fix(indexing): fail incomplete content indexing

[llbbl]

Closes #75

Summary

• Extract testable indexing orchestration out of the CLI entrypoint.
• Return exit code 0 only when indexing completes without failures.
• Keep progress logging and the final indexing summary intact while making partial and thrown failures stop downstream build work.

Changes

Indexing

• scripts/index-content-runner.ts: Adds a focused indexing runner that returns 0 for full success and 1 for partial results or thrown indexing errors.
• scripts/index-content.ts: Uses the runner and assigns process.exitCode so CI's existing index-before-build ordering stops before build when indexing is incomplete.

Tests

• scripts/index-content-runner.test.ts: Covers successful indexing, partial failures, and thrown indexing errors while preserving progress and summary logging.

Validation

• pnpm format
• pnpm lint
• pnpm exec tsc --noEmit
• pnpm test:coverage --run (165 tests; 94.48% overall line coverage)
• pnpm db:init:local
• pnpm index:local (3/3 documents)
• env TURSO_DB_URL=file:local.db TURSO_AUTH_TOKEN=local pnpm build

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.33%. Comparing base (73e8f84) to head (c0f7095).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #92   +/-   ##
=======================================
  Coverage   89.33%   89.33%           
=======================================
  Files          12       12           
  Lines         300      300           
  Branches       71       71           
=======================================
  Hits          268      268           
  Misses         16       16           
  Partials       16       16           

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