AK-508: add .gitignore for agent review-findings dirs

[benw5483]

Summary

• Adds a .gitignore to this repo, which had none, so automated review tooling's findings directories can never be committed here.
• Ignores .architect/ and .reviewer/, the gate-findings directories agent tooling writes at the repo root. They carry internal run identifiers and raw review commentary, so a broad git add -A must not sweep them into a PR.
• Adds .DS_Store as the ordinary baseline. No node_modules/ or Python entries, because this repo tracks only Markdown, JSON, YAML, one shell script, and one image. Neither language is present.

Verified before committing

git ls-files | grep -E '^\.(architect|reviewer)/' returns nothing on main, so no findings file was ever committed to this repo. The new rule is purely preventive and untracks nothing.

The diff is one new file and eight added lines. Nothing is removed, reordered, or otherwise touched.

Test plan

• git check-ignore -v .architect/round-1.findings.json .reviewer/round-1.findings.json names .gitignore as the matching source for both paths.
• git ls-files | grep -E '^\.(architect|reviewer)/' is empty, confirming nothing matching was already tracked.
• Create a scratch .architect/ directory in a clone, then confirm git status --porcelain stays clean.

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Generated by the operator's software factory.
• City: factory-main · Agent: local-core__builder-fm-session-c20ba9dca735092c301bd842f4e87b44
• On behalf of: @benw5483