[Linux2010]

Description

Adds a second agent skill to the repo's rule layer, plus the router line that makes it loadable. Proposed and discussed in #1413.

AGENTS.md routes code style to docs/ai/coding_conventions.md, the pre-done gate to docs/ai/mandatory_checks.md, and mistakes to mistake-reflection — but nothing states which surfaces a task-state write must reach. That missing link keeps producing the same defect class: a value that is correct on one surface and missing or stale on another.

This PR adds .agents/skills/task-state-invariants/SKILL.md, in the same shape as mistake-reflection (trigger list → procedure → anti-patterns):

• the invariant set, each claim anchored to current main: terminal states require status.timestamp (utils/constants.py:35 → database_task_store.py:152-157); stamp the event before CopyFrom, not the copy after (task_manager.py:285); CopyFrom inherits the previous timestamp, so explicit transitions such as the cancel path (active_task.py:971-973) must restamp; push delivery is best-effort (active_task.py:409); zero events means an unbounded wait (default_request_handler_v2.py:518-520 → active_task.py:1115-1117); store and stream must agree; v0.3 compat has two independent mappers (compat/v0_3/proto_utils.py vs conversions.py); continuation identity is inferred per spec §3.4.3;
• a sibling-surface checklist to run before declaring such a change done;
• a test map naming the existing file per surface, so coverage lands where the surface is;
• the evidence a PR in this area should carry, consistent with docs/ai/evidence_rules.md.

Also adds one numbered item to AGENTS.md → "Mandatory workflow". Without it the skill is unreachable: the rule layer scan shows AGENTS.md is the only file in the repo that links into .agents/skills/.

Scope is documentation only — no runtime code, no public API, no behavior change. If maintainers would rather the rule text live in docs/ai/ with a thin skill pointer, or want the AGENTS.md line dropped, say so and I will reshape it.

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title in the https://www.conventionalcommits.org/ specification.
• Ensure the tests and linter pass (Run bash scripts/format.sh from the repository root to format)
• Appropriate docs were updated (this PR is the documentation)

Fixes #1413 🦕

Validation

• Claim: the mandatory check sequence passes on this branch, and the skill loads under the same frontmatter contract as mistake-reflection.
• Exact head: 2ab36de
• BEFORE: .agents/skills/ contained one skill (mistake-reflection) and AGENTS.md had one route into it; nothing documented sibling-surface propagation. AFTER: two skills, two routes, no code change (git diff --name-only lists only AGENTS.md and the new SKILL.md).
• ./scripts/lint.sh: SUCCESS — "All linting and formatting tasks complete", ty reports only the 3 pre-existing unused-ignore-comment diagnostics that are present on clean main as well.
• uv run pytest: 2520 passed, 207 skipped, 4 xfailed, 1 xpassed. The 4 tests/integration/cross_version errors are environmental, not caused here: stashing this branch's changes and re-running on clean main reproduces the identical 4 errors (AssertionError: Server 0.3 HTTP failed to start — the sandbox cannot bind the subprocess server).
• Every file:line in the skill was re-read from current main at this head rather than copied from older notes; the anchors that changed since I first drafted them (a duplicated TERMINAL_TASK_STATES that has since been centralized in utils/constants.py:35) were corrected before committing.

Prepared with AI assistance (WorkBuddy); reviewed and verified by Linux2010