feat(rules)!: retire rulebook subscriptions and per-project suppressions (#4052)

A rule's home is its scope now: a rule in a rulebook topic is global, a rule on
a project applies to that project, and retrieval reads that directly (#4074).
A subscription had stopped changing anything a session received; a suppression
muted rules from a subscription. Operator, 2026-09-15: "we have global and
project scoped rules, we don't need the subscriptions now."

What goes, whole (rule 22):
- Migration 0101 drops project_rulebook_subscriptions, project_rule_suppressions
  and project_topic_suppressions, and strips subscribe_rulebooks (and 394's
  leftover exclude_always_on_rulebooks) from stored inception choices.
- Service, MCP and REST: subscribe/unsubscribe and the four suppress/unsuppress
  operations. The Subscribers checklist, the subscribe chips, the skip buttons
  and the Suppressed section in the rules UI.
- Inception asks two questions (design system, seed Systems). create_project and
  decide_project_inception lose subscribe_rulebooks.
- Backup v15 stops exporting the three sections; older archives still restore,
  the keys simply unread. Trash no longer hard-deletes suppression rows.

What changes meaning:
- get_applicable_rules is a project's LISTING: its own rules, plus the global
  rules tagged to an area it works in. Untagged global rules apply everywhere
  and arrive by retrieval, so they are not listed. A co_surfaces partner on a
  different project is not dragged in.
- list_rules(project_id) lists that project's own rules.
- rules_payload drops subscribed_rulebooks and suppressed_*; the handshake's
  brief form is project_rules alone.
- using-scribe's "Where a new rule goes" and inception sections, tool
  docstrings and docs say global vs project. Plugin 2026.09.15.1620.

Milestone 414 step 2.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/models/rulebook.py

@@ -236,36 +236,10 @@ class RuleRelation(Base, CreatedAtMixin):
         }
 
 
-# Pure many-to-many — no model class, just the join table.
-project_rulebook_subscriptions = Table(
-    "project_rulebook_subscriptions",
-    Base.metadata,
-    Column("project_id", BigInteger, ForeignKey("projects.id", ondelete="CASCADE"), primary_key=True),
-    Column("rulebook_id", BigInteger, ForeignKey("rulebooks.id", ondelete="CASCADE"), primary_key=True),
-    Column("created_at", DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)),
-)
-
-# Suppressions — let a project mute individual rules or whole topics from
-# rulebooks it subscribes to, without unsubscribing the rulebook itself.
-# FKs CASCADE so the row vanishes when its parent is removed.
-project_rule_suppressions = Table(
-    "project_rule_suppressions",
-    Base.metadata,
-    Column("project_id", BigInteger, ForeignKey("projects.id", ondelete="CASCADE"), primary_key=True),
-    Column("rule_id", BigInteger, ForeignKey("rules.id", ondelete="CASCADE"), primary_key=True),
-    Column("created_at", DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)),
-)
-
-# `project_rulebook_exclusions` lived here until milestone 394. It recorded a
-# project's opt-out of a whole always-on rulebook — which only made sense
-# while a rulebook could bind a project WITHOUT being asked. Subscription is
-# now the only reach a rulebook has, so declining one is expressed by not
-# subscribing, and there is nothing left to opt out of.
-
-project_topic_suppressions = Table(
-    "project_topic_suppressions",
-    Base.metadata,
-    Column("project_id", BigInteger, ForeignKey("projects.id", ondelete="CASCADE"), primary_key=True),
-    Column("topic_id", BigInteger, ForeignKey("rulebook_topics.id", ondelete="CASCADE"), primary_key=True),
-    Column("created_at", DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)),
-)
+# `project_rulebook_subscriptions`, `project_rule_suppressions` and
+# `project_topic_suppressions` lived here until milestone 414 (migration 0101).
+# A rule's home is its scope now: a rule in a rulebook topic is global, a rule
+# on a project applies to that project, and retrieval reads that directly. A
+# subscription had stopped changing anything a session received, and a
+# suppression muted rules from a subscription. A project that departs from a
+# global rule writes a project rule with an `overrides` relation instead.