docs: record migration head 0020 in the release manifest and docs

RELEASE_MANIFEST.json still said head 0011 (repo was at 0018, now 0020)
and RELEASE_CANDIDATE_V1.md said 0010. Updated both, plus
AI_PROJECT_STATE.md and PILOT_DEPLOYMENT.md, to head
0020_not_null_timestamps as applied in production.

scripts/release_manifest.py reported the 4-digit file prefix as the head
("0020" instead of "0020_not_null_timestamps") and always read the
applied revision from the dev container. It now asks Alembic for the real
head and revision count, and the Hub container is configurable
(HUB_CONTAINER); the source is recorded in the manifest.

tests/test_release_candidate.py now fails when the manifest head differs
from Alembic's head.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PILOT_DEPLOYMENT.md

@@ -130,6 +130,10 @@ Agents with `central/scripts/register_agent.py` (see below).
 - `0019` repairs `agents.created_at/updated_at` and `jobs.created_at`, whose
   defaults `0006` had frozen at the instant it ran (rows created before `0019`
   keep that frozen value — their real creation time is not recoverable).
+- `0020` enforces NOT NULL on 20 timestamp columns that always had
+  `DEFAULT now()`. It aborts, changing nothing, if any NULL exists — clean
+  those rows explicitly first. Current head: `0020_not_null_timestamps`
+  (also recorded in `release/RELEASE_MANIFEST.json`).
 
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