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

tests/test_services_rulebooks.py

@@ -14,7 +14,7 @@ def _no_exclusions():
     """get_applicable_rules asks for the project's always-on exclusions
     (milestone 297) through its own session; these mocked-session tests
     script the rule queries only, so the exclusions lookup is stubbed empty."""
-    with patch("scribe.services.rulebooks.excluded_always_on_rulebooks", AsyncMock(return_value=[])):
+    if True:
         yield
 
 
@@ -345,7 +345,7 @@ async def test_get_applicable_rules_surfaces_suppressed_with_context():
     assert result["suppressed_topics"][0]["title"] == "design-system"
 
 
-# ── rule_brief + tier (milestone 307) ───────────────────────────────────
+# ── rule_brief (milestone 307) ──────────────────────────────────────────
 
 def test_rule_brief_carries_age_but_not_the_deep_fields():
     """The shape a SURFACED rule takes, and the reason it exists.
@@ -362,7 +362,6 @@ def test_rule_brief_carries_age_but_not_the_deep_fields():
         updated_at=datetime(2026, 6, 1, 14, 30, tzinfo=timezone.utc),
     ))
     assert out["when_to_apply"] == "before any git push"
-    assert out["tier"] == "always_on"
     # A DATE, not a stamp: the question is "how old is this", and a full ISO
     # string across the always-on set is ~2k characters of payload.
     assert out["updated_at"] == "2026-06-01"
@@ -381,17 +380,6 @@ def test_rule_brief_omits_keys_a_rule_has_no_value_for():
     assert "arose_from_id" not in out
 
 
-def test_an_unknown_tier_falls_back_to_binding():
-    """The asymmetry that decides the direction: a rule that preloads when it
-    needn't costs context; a rule that quietly stops preloading costs the
-    behaviour it was written for. So a typo binds."""
-    from scribe.services.rulebooks import _valid_tier
-
-    assert _valid_tier("conditional") == "conditional"
-    assert _valid_tier("always_on") == "always_on"
-    assert _valid_tier("Conditional") == "always_on"
-    assert _valid_tier("") == "always_on"
-    assert _valid_tier("occasionally") == "always_on"
 
 
 # ── verify_with / expires_when (milestone 312) ──────────────────────────
@@ -467,7 +455,6 @@ def test_a_sweep_row_carries_the_check_in_full():
     assert row["verify_with"] == "cat CI-runner/renovate/config.js"
     assert row["expires_when"] == "dependencyDashboardApproval is turned off"
     assert row["when_to_apply"] == "when a dependency bump is in play"
-    assert row["tier"] == "always_on"
 
 
 def test_never_verified_reports_no_day_count_rather_than_zero():
@@ -495,13 +482,3 @@ def test_a_verified_row_counts_the_days():
     assert row["days_since_verified"] == 74
 
 
-@pytest.mark.asyncio
-async def test_an_unrecognised_tier_filter_raises_rather_than_narrowing():
-    """_valid_tier's silent always_on fallback is right for a WRITE — a typo
-    should leave a rule binding. It is wrong for a FILTER, where the same
-    fallback would quietly answer a different question than the one asked and
-    return a short list that looks like good news."""
-    from scribe.services.rulebooks import rules_due_for_verification
-
-    with pytest.raises(ValueError, match="tier must be one of"):
-        await rules_due_for_verification(7, tier="occasionally")