Reclaim rebuild-created SQLite free pages

[Widthdom]

Summary

• reclaim rebuild-created SQLite free pages after successful CLI and MCP rebuild commits when the shared maintenance threshold recommends it
• use bounded incremental vacuum only for compatible databases, preserving the committed index on skip, failure, cancellation, and concurrent-reader scenarios
• expose immediate and persisted before/after reclaim telemetry, including truthful unavailable values when post-reclaim measurement fails
• document the policy and cover integrity, interruption, explicit vacuum, logical size attribution, and memory-timeline ordering

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore — 0 warnings, 0 errors
• Issue Avoid leaving hundreds of megabytes of free pages after a rebuild #5057 and memory-timeline focused tests — 8 passed on net8.0 and 8 passed on net9.0
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore --no-build — 21,997 passed, 420 skipped, 0 failed
• dotnet run --project tools/CodeIndex.Changelog -- check — validated 6 fragments
• local cdidx status --json --check=workspace — workspace matched, no failed checks
• Codex adversarial review — 2 rounds completed; all three P2 findings were addressed, with behavioral fixes covered by regression tests

Documentation and changelog

• Updated README.md, USER_GUIDE.md, DEVELOPER_GUIDE.md, TESTING_GUIDE.md, and AGENT_GUIDE.md
• Added bilingual changelog.d/unreleased/5057.fixed.md

Related to #1631.

Fixes #5057