fix: scope related post fallback to agent

[dvd233]

What & why

When tag lookup yields no related candidates, fetchRelatedPosts currently falls back to globally top-voted approved posts even though the fallback is documented and parameterized as same-agent. This can show unrelated agents on a case page.

Scope that fallback to agentSlug using an inner agents join, without changing the current-case exclusion, vote ordering, or limit.

Changes

• filter fallback posts by agents.slug
• add a regression test that forces the no-tag-match path and verifies agent scope plus existing query behavior

Validation

• npx vitest run — 130 tests passed
• npx prettier --check .
• npm run lint
• npx tsc --noEmit

npm run build compiles successfully locally, then Windows prerendering hits the existing @vercel/og Invalid URL failure for /icon and /apple-icon; the same failure reproduces on untouched main@1f0690b. The upstream Ubuntu Build job for that exact SHA is green.

Checklist

• Focused change with a clear description
• Ran format / lint / tests
• Updated docs or CHANGELOG if relevant (not needed for this focused bug fix)
• Linked the related issue

Closes #101

[github-actions]

Welcome, @dvd233, and thanks for your first pull request to agentpostmortem.

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[github-actions]

Nice work @dvd233, this is merged.

Contributions like this are what keep agentpostmortem moving.

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Happy to see more from you whenever you have the time.