fix: exclude Meta sites from content scripts

[solomonneas]

Summary

• exclude Facebook, fbcdn, and Instagram URLs from always-on extension content script injection
• share the exclusion list across extract, hover, and automation content scripts
• add a manifest regression test that verifies the generated content_scripts exclusions

Fixes #106.

Verification

• pnpm -C apps/chrome-extension test:chrome (51 passed, 3 skipped)
• pnpm -C apps/chrome-extension build:firefox
• inspected Chrome and Firefox manifests: both include the Meta exclude_matches list
• pnpm -s lint
• pnpm -s typecheck
• pnpm -s test:coverage (1731 passed, 40 skipped)

Note

• pnpm -s check currently stops at format:check because existing .claude/memory-handoffs/*.md files are not formatted by repo rules; I did not rewrite those handoff files for this PR.