Share FTS optimization recommendations

[Widthdom]

Summary

• Share one persisted FTS optimization recommendation across status, explain, standalone optimize --dry-run, index --optimize --dry-run, execution, and vacuum maintenance output.
• Keep the threshold, observed-write count, reason, freshness state, and recommended command consistent without adding writes to status paths.
• Preserve explicit immutable URI semantics in both dry-run entry points and suppress optimize command selection when higher-priority WAL or freelist maintenance state is unknown.

Root cause

Status maintenance guidance and optimize preview evaluated different inputs and thresholds, while execution used its own counter reads. The dry-run entry points could also normalize an explicit immutable URI before opening their query connection.

Validation

• Debug and Release builds: 0 warnings, 0 errors
• Issue-focused tests: 12/12 passed on both .NET 8 and .NET 9
• Full Release suite (.NET 8): 10,897 passed, 7 skipped, 0 failed
• Full Release suite (.NET 9): 10,423 passed, 420 skipped, 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: all findings resolved; final review reported no actionable issues

Documentation and changelog

• Updated the English and Japanese sections of README, DEVELOPER_GUIDE, and TESTING_GUIDE, plus the repository guidance in AGENT_GUIDE.
• Added changelog.d/unreleased/4887.fixed.md; CHANGELOG.md remains unchanged.

Fixes #4887