stigmem-node: decay sweep expires and counts facts across all tenants (cross-tenant BOLA)

Summary

### Summary On a multi-tenant stigmem node, a caller holding a `write` credential for **one** tenant can run a decay sweep that acts on **every** tenant's facts. The candidate-selection queries in `lifecycle/decay.py` (`_select_ttl_candidates`, `_select_confidence_candidates`) carried no `tenant_id` predicate, and the caller's tenant was not threaded into the sweep or its async worker (`run_decay_sweep` / `_decay_job_worker`), reached via `POST /v1/decay/sweep`. ### Impact A sweep with `ttl_seconds=0` expires **all tenants'** facts — cross-tenant data destruction (integrity and availability). A `dry_run` sweep returns a global candidate count, acting as a cross-tenant existence/volume **oracle** (information disclosure). ### Affected configurations This is a cross-**tenant** break. It is exploitable **only** on deployments running the opt-in `stigmem-plugin-multi-tenant` (multiple tenants on one node). A default single-tenant node has only `tenant="default"` — there is no second tenant to cross — so it is **not exploitable** on default deployments. The rating is HIGH for the multi-tenant deployments the plugin exists to isolate. ### Patches Fixed in `0.9.0a12` (PR #728): `identity.tenant_id` is threaded into `run_decay_sweep` and `_decay_job_worker`, and `AND tenant_id = ?` was added to the candidate selectors and the graph-sync. A tenant-B sweep now leaves tenant-A facts untouched, and `dry_run` counts only the caller's tenant. The `check_fact_query_tenant_scope.py` CI guard was extended to scan `lifecycle/` so this class cannot silently regress. ### Workarounds None other than upgrading to `0.9.0a12`. Single-tenant deployments are unaffected.