Review: publish all memos by default (owner policy)

Summary

Implemented the owner's policy that all memos should be published by default, since the source code is already public on GitHub.

Changed Files

1. src/lib/memos.ts -- Updated scanAllMemos() to scan all 3 subdirectories (inbox, active, archive) instead of just archive. Changed the filter from data.public !== true (opt-in) to data.public === false (opt-out).
2. docs/memo-spec.md -- Updated the public field description to reflect the new default-public policy.

What Was NOT Changed

• No memos had public: false in frontmatter -- The task mentioned removing public: false from two memo files, but inspection confirmed neither file has public: false in its YAML frontmatter. The references to public: false in those files are in the body content (plan and review text), not in frontmatter fields.
• No test file existed -- src/lib/__tests__/memos.test.ts does not exist, so there were no tests to update.

Validation

• npx tsc --noEmit passes
• npm run build succeeds (118 static pages generated)
• npm run test passes (239/239 tests)
• Commit: cdc1117 with message feat(memos): publish all memos by default (owner policy)

Review Focus Areas

1. Verify the filter logic change from data.public !== true to data.public === false is correct
2. Verify the scan scope expansion to include inbox and active directories is appropriate
3. Confirm the memo-spec documentation update accurately reflects the new policy
4. Note: Two memos are being excluded by secret pattern detection (false positives on example text in review/plan memos) -- this is a pre-existing issue, not introduced by this change