Remove the NotebookLM hooks and workflows: the sync is a stub and the gate checks a tracked constant #5957

Owner decision 2026-10-04: remove NotebookLM from the git hooks and from CI ("do it").

What runs today, and what it does

The sync is a stub. contrib/backend/notebooklm/sync.py uploads placeholder text instead of the thing it is named after:

• sync.py:334 (handle_issue_comment): "In production, would fetch actual comment"
• sync.py:343 (handle_pr_event): "In production, would fetch PR diff and comments"
• sync.py:55 reads sync_state.json; no such file exists in the tree.

CI swallows every failure of it.

• .github/workflows/notebook-sync.yml:178-181: python3.10 sync.py "${SYNC_ARGS[@]}" || { echo "Sync completed with warnings"; exit 0; }
• .github/workflows/notebook-sync.yml:273: python3.10 contrib/backend/notebooklm/sync.py --activity || true
• .github/workflows/notebook-sync.yml:183-241 (Update sync status as commit (best-effort), continue-on-error: true) tries to commit .trinity/notebook_sync_status.json to the default branch. The ruleset refuses that, and the step logs a warning and stays green.

Run 37183036041 (PR #5890) shows both: the sync step prints Syncing PR merge for #5890 and nothing after it. The next step logs Status commit skipped (409 Repository rule violations found ... Changes must be made through a pull request.). The job is green.

The last 100 notebook-sync.yml runs (2026-10-04 05:53Z to 07:51Z) are 84 success and 16 in progress, with 0 failures. A workflow that cannot fail tells nobody anything.

The gate checks a tracked constant.

• .github/workflows/notebook-gate.yml:43-74 and .githooks/pre-push both pass if .trinity/current_task/.notebook_id exists and matches ^[a-zA-Z0-9]{8,}$.
• That file is tracked in git. It holds b83263109fb055dc and was last changed in ea15cd5 (conformance: promote gf256 -> strict SW-bitexact (last rung, 75/0/8) #1399). Every checkout carries it, so the gate passes for every PR and every push, whatever the change.
• The last 100 notebook-gate.yml runs are 89 success, 2 cancelled and 9 in progress, with 0 failures.
• The fix the pre-push hook offers is t27c bridge task start. bootstrap/src/bridge.rs:427-428 makes "a fake notebook ID for now" of the form nb-<title>, and the hyphen fails the gate's own regex.
• The pre-push hook also offers a bypass, SKIP_NOTEBOOK_GATE=1.

The hooks start the stub on the developer's machine.

• .githooks/pre-commit (the NotebookLM block after tri hooks pre-commit) bumps a commit counter under the git common dir and, every third commit or when a .t27 file is staged, starts python3.10 sync.py in the background with >/dev/null 2>&1 || true.
• .githooks/post-merge does the same after every merge.

Neither workflow is a required context. The ruleset (gh api repos/gHashTag/t27/rules/branches/master) requires validate, check-linked-issue and parse-ratchet.

Decision

Remove the NotebookLM part of the hooks and both workflows:

1. .githooks/pre-commit: remove the NotebookLM block; keep tri hooks pre-commit.
2. .githooks/pre-push: the whole file is the gate, so delete it.
3. .githooks/post-merge: the whole file is the sync, so delete it.
4. Delete .github/workflows/notebook-sync.yml and .github/workflows/notebook-gate.yml.
5. Remove what only existed for these: the SKIP_NOTEBOOK_GATE bypass, instructions that say a notebook must exist, and the CI test whose only subject is the branch-to-issue parser in the two hooks.

Out of scope, left for a follow-up: contrib/backend/notebooklm/, .trinity/current_task/.notebook_id and the local .trinity/notebook_commit_count. Once nothing calls them they are dead.

Stacked on #5951 (removes the NOW gate from the same hooks). Refs #5906.