feat(rules)!: retire the always-on tier — every rule arrives by retrieval (#394)

Milestone 394, steps 5-8. Operator: "remove the always on rule functionality
as the goal was to not have it at all since it didn't seem to work as
expected."

Unconditional preload had three failures the retrieval arms do not. It could
not be MEASURED — a resident rule is in the context whether or not it
mattered, so nothing distinguished "this governed the act" from "this was
scenery", and it was the one surface structurally exempt from the scoreboard
judging every other. It was SUMMARISED AWAY by compaction while the session
went on believing it held the rules. And it CROWDED OUT the few rules that
applied with the thirty that did not.

WHAT GOES

Schema (0100): rules.tier + ck_rules_tier, rule_versions.tier,
rulebooks.always_on, and project_rulebook_exclusions — a table recording a
project's opt-out of something that no longer binds it unasked.

Tools: list_always_on_rules, exclude_always_on_rulebook,
include_always_on_rulebook. Service: the same three plus rules_etag_for,
_valid_tier and the whole etag family. The SessionStart preload and the
write-path staleness arm go with them: nothing is resident, so nothing can
have drifted since a session loaded it.

THREE CALLS WORTH REVIEWING

enter_project got NARROWER, not wider. Its filter was `always_on OR
area-tagged`; dropping the tier arm leaves the deterministic half, so a
project with no canonical-tagged Systems gets no bulk rules and reaches them
by retrieval instead. Dropping the whole clause would have made that payload
bigger than the preload this milestone deletes.

Backups import tolerantly. A pre-394 archive carries tier, always_on and the
retired inception choice; none is read, and the exclusion key is DROPPED
rather than remapped, because restoring it would write data that
validate_inception now rejects as unknown.

The migration is irreversible in the way that matters and says so: downgrade
recreates the columns at their defaults and cannot restore which rules were
always-on. A value invented to fill a hole is not a measurement.

THE INSTRUCTION SURFACES SAY THE HARDER THING

Deleting "call list_always_on_rules()" is easy; replacing it is not, because
the new model asks a session to trust something it cannot see. All three
surfaces now say a session holds nothing, that rules arrive when work matches
them, and — the half that got dangerous — that "no rule arrived" means
"nothing matched", never "there is no rule". Under residency an empty session
was rare and suspicious; it is now the ordinary state of most turns, so
reading it as permission is wrong on nearly every turn rather than
occasionally. That is #3720's defect at session scale.

test_instruction_surfaces_agree is repointed rather than retired: its two
halves collapsed into one instruction, and it gains a guard that every
surface states what absence means. _INSTRUCTIONS is back at 1999/2000 —
the inception clause paid for the longer HOW line.

UI (rule 27, and the opportunity step 8 named)

The tier selector is gone, and what replaces it is the point: `when_to_apply`
is now the field that decides whether a rule is ever seen, so the editor
marks it required, warns while it is empty, and both rule lists badge a
trigger-less rule "never surfaces". A rule without one is not quiet, it is
unreachable.

TESTS

Two files deleted outright — test_rules_etag.py and test_inception_rules.py
tested subsystems that no longer exist. Elsewhere obsolete cases were removed
and the rest repointed. One deserves naming: the wiring test asserted the act
arms pass no `tier`, which had become an assertion that could not fail. It is
repointed onto `kind`, which does still exist and where the same claim is
live — a preference must reach a write exactly as a rule does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011cPyzNnegXHr5iRMzzy5KJ

alembic/versions/0100_drop_always_on_tier.py

@@ -0,0 +1,96 @@
+"""drop the always-on tier: rules.tier, rulebooks.always_on, the exclusions table
+
+Revision ID: 0100
+Revises: 0099
+Create Date: 2026-09-11
+
+Milestone 394. Every rule now reaches a session by retrieval — because
+something it is about to do made the rule relevant — and the machinery that
+delivered rules unconditionally goes with it.
+
+WHAT GOES, AND WHERE IT CAME FROM
+
+  - ``rules.tier`` and its ``ck_rules_tier`` CHECK (migration 0088). Dropping
+    the column takes the constraint with it. Rule 36 is about ADDING a value to
+    a live whitelist, which needs DROP + ADD in the same migration; it does not
+    speak to removing the column outright, and saying so here is cheaper than
+    the next reader wondering whether it was forgotten.
+  - ``rule_versions.tier`` (migration 0098). A version records what a rule
+    SAID; with no tier on a rule there is nothing for a snapshot to carry.
+  - ``rulebooks.always_on`` (migration 0058). A rulebook reaches a project by
+    subscription now, and by nothing else.
+  - ``project_rulebook_exclusions`` (migration 0085). It recorded a project's
+    opt-out of an always-on rulebook. Opting out of something that no longer
+    binds you is not a state that can exist — declining a rulebook is
+    expressed by not subscribing to it.
+
+IRREVERSIBLE, AND THE DOWNGRADE SAYS SO RATHER THAN PRETENDING
+
+The downgrade recreates the columns and the table with their DEFAULTS. It
+cannot restore WHICH rules were always-on, which rulebooks bound every project,
+or which projects had opted out — that information is in what this drops.
+
+That distinction is the one this repo keeps insisting on: a value invented to
+fill a hole is not a measurement. So a downgraded database is structurally able
+to run the old code and is NOT the database the old code was running against —
+every rule comes back at the ``always_on`` default, which for the tier column
+happens to mean "binding", the safe direction to be wrong in.
+
+Anyone who needs the real prior state restores a backup taken before this ran.
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0100"
+down_revision = "0099"
+branch_labels = None
+depends_on = None
+
+_TIERS = ("always_on", "conditional")
+
+
+def _in_list(column: str, values: tuple[str, ...]) -> str:
+    return f"{column} IN (" + ", ".join(f"'{v}'" for v in values) + ")"
+
+
+def upgrade() -> None:
+    op.drop_table("project_rulebook_exclusions")
+    op.drop_column("rulebooks", "always_on")
+    op.drop_column("rule_versions", "tier")
+    # The CHECK goes with the column it constrains; naming it here would be a
+    # second drop of the same object.
+    op.drop_column("rules", "tier")
+
+
+def downgrade() -> None:
+    """Structure only. See the module docstring — the values are gone."""
+    op.add_column(
+        "rules",
+        sa.Column("tier", sa.Text(), nullable=False, server_default="always_on"),
+    )
+    op.create_check_constraint("ck_rules_tier", "rules", _in_list("tier", _TIERS))
+    op.add_column("rule_versions", sa.Column("tier", sa.Text(), nullable=True))
+    op.add_column(
+        "rulebooks",
+        sa.Column(
+            "always_on", sa.Boolean(), nullable=False,
+            server_default=sa.text("false"),
+        ),
+    )
+    op.create_table(
+        "project_rulebook_exclusions",
+        sa.Column(
+            "project_id", sa.BigInteger(),
+            sa.ForeignKey("projects.id", ondelete="CASCADE"),
+            primary_key=True, nullable=False,
+        ),
+        sa.Column(
+            "rulebook_id", sa.BigInteger(),
+            sa.ForeignKey("rulebooks.id", ondelete="CASCADE"),
+            primary_key=True, nullable=False,
+        ),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=True,
+        ),
+    )