chore: ignore local .serena/ Serena MCP config

[rldyourmnd]

Summary

Add /.serena/ to the root .gitignore. Serena MCP auto-generates
.serena/project.yml and .serena/.gitignore as local semantic-code
tooling config. This keeps that developer-local scaffold out of the published
reusable-workflow catalog — the repo intentionally carries no AI-tool footprint.
Only the ignore rule changes; no product, workflow, catalog, or docs surface is
touched.

Type of change

• None of the listed types — repository hygiene (.gitignore only). Not a
  workflow / capability / tool-bump / hardening / docs change.

Threat-model note

N/A — no workflow behavior, token scopes, egress, or which-actions-run changed.
A single .gitignore line.

Permissions diff

N/A — no permissions: block touched.

Checklist

• No third-party uses: changed (N/A — no workflow touched).
• No permissions: changed (N/A).
• No workflows/jobs changed → concurrency / timeout-minutes N/A.
• No actions/checkout changed (N/A).
• No run: blocks changed (N/A).
• Dual-tier toggles untouched (N/A).
• Repo self-check python3 scripts/validate_all.py → OK (all 9 checks green); actionlint / zizmor --pedantic N/A (no workflow change).
• README / docs/ / catalog/ — N/A (no workflow added/changed).
• CHANGELOG.md [Unreleased] — intentionally not updated: this is local repo hygiene with no user-facing catalog/workflow impact. Happy to add a line if you prefer.
• [~] Commits: DCO signed-off (-s) + Conventional Commit. Not GPG/SSH-signed locally (signing is unset on this machine). The required_signatures rule on main will be satisfied by GitHub's signed squash-merge.

Tier impact

• None — repository hygiene only; affects no billing tier.