docs(AGENTS.md): add post-merge lint workflow to Common mistakes

[scottschreckengaust]

Problem

After merging main into a feature branch, files introduced on main may contain lint suppressions that are stale under the feature branch's config. If the auto-fix output isn't committed, CI's "Fail build on mutation" step rejects the PR.

This happened on PR #171 (ESLint 10 upgrade): after merging main (which included PR #176 attachments), two files had // eslint-disable-next-line comments for rules not enabled in the new flat config. ESLint --fix removes them, but the removal wasn't committed — causing a CI mutation failure.

Findings

1. The root cause is a workflow gap: merging main into a config-change branch brings in code that hasn't been processed by the new config. The merge itself doesn't run linters.

2. The fix pattern: After any merge of main into a feature branch, run mise //cdk:eslint and mise //cli:eslint (which use --fix), then commit any resulting changes.

3. Where to document: AGENTS.md → "Common mistakes" section, adjacent to the existing Git worktrees bullet. This is where agents read workflow rules on every session start.

4. The broader workflow insight: The complete development workflow is:

   issue → worktree → branch → local checks → PR draft → remote checks → PR ready for review
   

   The "local checks" step must include running linters with --fix after merges — not just verifying zero errors. CI checks for mutations (uncommitted auto-fixes), so local development must capture and commit those mutations before pushing.

Proposed addition to AGENTS.md

In the "Common mistakes" section, after the "Git worktrees" bullet:

- **After merging main into a feature branch** — Always run **`mise //cdk:eslint`** and **`mise //cli:eslint`** (which use `--fix`), then commit any resulting changes. Merged-in files may contain stale lint suppressions that the feature branch's config removes. CI's "Fail build on mutation" step will reject uncommitted auto-fixes.

Acceptance criteria

• AGENTS.md updated with post-merge lint guidance
• Starlight mirrors regenerated (mise //docs:sync)
• PR passes CI (no mutations)

References

• PR feat(tooling): upgrade ESLint 9 → 10 with flat config migration #171 CI failure: https://github.com/aws-samples/sample-autonomous-cloud-coding-agents/actions/runs/26474172265/job/77955251211
• Fix commit: 1fd3127 on feat/eslint-10-upgrade