docs(planning): file phase-3 task 3.5 — retire processor migration runner

Replaces the original "migration advisory lock" sketch. Once processor
doesn't run DDL, the lock concern delegates to Directus's db-init runner.

Context: positions hypertable + faulty column DDL currently exists in
both processor (src/db/migrations/0001 + 0002) and directus
(db-init/001/002/003). Two sources of truth for the same schema is a
known hazard — adding a column means editing two files in two repos,
and silent drift between them is invisible until runtime.

Fix: directus becomes the sole DDL owner. Processor's migration runner
is retired; only INSERT/SELECT/UPDATE remain.

Task spec covers:
- Pre-flight diff between processor migrations and directus db-init
  (must be byte/semantically equivalent before deletion)
- File-by-file deletion list
- Test infra migration (integration test moves to fixture-based schema
  setup, matching the established Phase 1.5 task 1.5.6 pattern)
- Wiki + ROADMAP updates
- compose.yaml depends_on directus: service_healthy
- Operational notes (existing migrations_applied table is left in place)

Sequence: ideally lands AFTER Phase 1.5 ships so the agent shipping the
WS endpoint isn't pulled into a side quest mid-flight.

.planning/phase-3-hardening/README.md

@@ -25,7 +25,7 @@ When Phase 3 is done:
 | 3.2 | Per-device state rehydration on first-packet | Single `SELECT ... LIMIT 1` per cold device. Memoized by LRU |
 | 3.3 | `XAUTOCLAIM` runner | Periodic + on-startup. Claims entries pending > `CLAIM_THRESHOLD_MS`. Re-runs the sink |
 | 3.4 | Dead-letter stream | After N failed decodes/writes, record goes to `telemetry:teltonika:dlq`; original ACKed off the main stream |
-| 3.5 | Migration advisory lock | `pg_advisory_lock(<hash>)` around the migrate runner; two instances can start simultaneously |
+| 3.5 | [Retire processor migration runner](./05-retire-migration-runner.md) | Delete `src/db/migrations/*` and the runner; Directus becomes the sole DDL owner via its `db-init/`. Closes the two-sources-of-truth hazard for `positions`. Replaces the original "migration advisory lock" sketch — once processor doesn't run migrations, the lock concern delegates to Directus. |
 | 3.6 | Uncaught exception / unhandled rejection handlers | Log, flush, exit. Match `tcp-ingestion`'s eventual Phase 1 task 1.12 work when that lands |
 | 3.7 | OPERATIONS.md | The runbook |
 | 3.8 | Multi-instance load test | A test (manual or in CI) that proves two instances split the work; document expected lag behaviour during failover |