feat(bug-fast-track): cheaper/shorter pipeline route for bug-fix tasks (ORCH-019)

A task carrying the Plane `Bug` label takes a shortened route that skips the
`architecture` stage (one opus architect run + ADR + check_architecture_done),
replacing heavy analysis with a lite package (bug-report + mandatory regression
test plan). EVERY Quality Gate / sub-gate runs UNCHANGED — the route is a
scheduler property, not a gate (root invariant NFR-1): STAGE_TRANSITIONS /
QG_CHECKS / check_* / machine-verdict keys are byte-for-byte preserved.

- src/bug_fast_track.py: new leaf (never-raise) — bug_fast_track_applies (local,
  network-free, checked first), is_bug_task (labels.has_label, Plane API source),
  skips_architecture (pure DB-backed routing predicate), snapshot.
- src/db.py: additive idempotent tasks.track column (TEXT DEFAULT 'full') +
  set_task_track / get_task_track helpers (missing/NULL -> 'full', fail-safe).
- src/stage_engine.py: routing-override on the analysis-exit edge (track='bug' ->
  development/developer, skipping architect); brd-review-clock stamp extended to
  analysis->development. get_next_stage/get_agent_for_stage stay pure.
- src/webhooks/plane.py: classify task as bug in start_pipeline (applies-first
  short-circuit; never-raise -> full cycle on any error).
- src/main.py: additive bug_fast_track block in GET /queue + POST
  /bug-fast-track/escalate (reset 'bug'->'full' to return to the full cycle).
- src/config.py: bug_fast_track_enabled / _label / _repos flags (empty CSV ->
  self-hosting only).
- src/notifications.py: optional 🐞 marker on the bug-track card (never-raise).
- Prompts: analyst.md (lite bug package + escalation), reviewer.md (regression-
  test axis) — 52d canon preserved.
- Docs: CLAUDE.md, README.md (env + API + section), docs/architecture/README.md,
  CHANGELOG.md, .env.example.
- Tests: tests/test_bug_fast_track*.py + test_db_migrations.py + queue block
  (TC-01..TC-15). Full regression green (1551 passed).

Kill-switch ORCH_BUG_FAST_TRACK_ENABLED=false -> 1:1 pre-ORCH-019 (zero
regression; residual track column harmless).

Refs: ORCH-019

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -139,6 +139,18 @@ ORCH_SERIAL_GATE_FREEZE_ENABLED=true
 #                                for enduro too).
 ORCH_STOP_STATUS_ENABLED=true
 ORCH_STOP_STATUS_REPOS=
+# ORCH-019: bug-fast-track — a cheaper/shorter pipeline route for bug-fix tasks.
+# A task carrying the Plane `Bug` label skips the whole `architecture` stage; EVERY
+# Quality Gate / sub-gate runs UNCHANGED (route is a scheduler property, not a gate).
+# Additive, never-raise, fail-safe -> full cycle. Infra precondition: create a `Bug`
+# label on the ORCH board (its absence = full cycle, fail-safe). Leaf src/bug_fast_track.py.
+#   BUG_FAST_TRACK_ENABLED=false -> start_pipeline AND advance_stage are 1:1 as before
+#                                   ORCH-019 (zero regression).
+#   BUG_FAST_TRACK_LABEL         -> Plane label that activates the track (default `Bug`).
+#   BUG_FAST_TRACK_REPOS (CSV)   -> scope; EMPTY = self-hosting only (orchestrator).
+ORCH_BUG_FAST_TRACK_ENABLED=true
+ORCH_BUG_FAST_TRACK_LABEL=Bug
+ORCH_BUG_FAST_TRACK_REPOS=
 # ORCH-094: terminal-window-aware guard for the three deploy-phase Plane status
 # setters (set_issue_awaiting_deploy / set_issue_deploying / set_issue_monitoring).
 # A DB stage=done task converges to Done idempotently instead of flapping