fix(kanban): gate autonomous dispatch by governance headers

[camaragon]

Summary

• gate Kanban autonomous promotion/claim/dispatch on top-of-body Approval: and Execution: headers
• allow only non-gated approvals (observe, propose, execute-safe) with worker/assigned execution to be spawnable
• add regression coverage for approval-required/self/hold/assigned mismatch behavior
• add cron prompt scanner regression so benign PR-maintenance secret-scan wording is allowed while real curl env exfil stays blocked

Test Plan

• python -m pytest tests/hermes_cli/test_kanban_db.py tests/tools/test_cron_prompt_injection.py -q -o 'addopts=' → 76 passed locally

CI note

Current PR CI has repository-baseline noise:

• test is red, but current main at 839cdd1b054a75ff1b581199a83488c8e0f2f788 is also red on the same lane.
• Compared failed-test summaries: PR has 0 PR-only failures; its 40 failed tests are a subset of main's 47 failed tests.
• ruff + ty diff failed only while posting/updating the PR lint comment with 403 Resource not accessible by integration; the lint report itself says ruff has 0 issues and ty diagnostics are warnings.

Risk

Low. Dispatcher gating is intentionally conservative for new governance headers while legacy cards without headers keep existing worker/execute-safe default behavior.

Notes

• Prepared in a clean worktree from current origin/main.
• No unrelated local dirty files included.
• No secrets found in diff scan.

[teknium1]

Closing — please reopen when a concrete use case lands.

The mechanics here are clean (header parser + gate at recompute_ready / claim_task / has_spawnable_ready / dispatch_once, with a sane backward-compat default of Approval: execute-safe + Execution: worker for legacy rows). The reason for closing is the speculative-infrastructure-no-consumer pattern: there's no current code path that lets a card spawn workers it shouldn't because of a missing approval header — the headers don't exist anywhere in the codebase or skills today, no UI sets them, no dashboard surfaces them, no skill documents them. Adding the parser + the enforcement simultaneously means the first user-authored Approval: approval-required card will silently get stuck in todo with no UX feedback; you really need the producer side (board UI, CLI to set headers, dashboard rendering of approval state, migration docs for existing boards) to land first or alongside.

Two carve-outs from this PR are worth shipping independently if you'd like:

• .github/workflows/lint.yml — gating the comment-post step on same-repo PRs and adding continue-on-error. This is a real CI hygiene fix (fork PRs 403 on the comment post). Worth its own small PR. The fork-PR fix from a different author (PR fix(kanban): dedupe kanban notifier delivery claims #22558's drive-by) just landed via PR fix(kanban): dedupe notifier delivery via atomic claim + rewind on failure (salvage #22558) #23401, so this specific hunk may now be redundant — worth a quick check before opening.
• tests/tools/test_cron_prompt_injection.py — the cron-scanner allowlist test additions. Independent of the governance work; could land as its own focused PR.

When the governance-header story has a concrete consumer (a real RFC for which UI sets headers, what the dashboard shows, how operators correct mis-headered cards), happy to revisit. Reopening this PR is welcome at that point — the parser + gate work is good and won't have rotted much.

Thanks @camaragon.