Bound persisted status diagnostics

[Widthdom]

Summary

• Bound persisted structured status metadata before managed-string materialization, with explicit JSON depth and semantic contract validation.
• Keep status usable when one subdocument is invalid by emitting stable bounded diagnostics, and sanitize persisted first-failure and recovery-hint text in human output.
• Share writer/reader collection limits and add boundary, read-only, round-trip, and output-integrity regression coverage.

Validation

• Release and Debug solution builds completed with 0 warnings and 0 errors.
• Full Release tests: net8.0 — 11,830 passed, 9 skipped, 0 failed; net9.0 — 11,350 passed, 426 skipped, 0 failed.
• Focused Issue Bound and sanitize persisted status diagnostics at read and human-output boundaries #5161 tests passed on both target frameworks (7/7 each); related bounded JSON, documentation-contract, snapshot, and projection tests passed on both frameworks (88/88 each).
• dotnet format CodeIndex.sln --verify-no-changes --no-restore passed.
• dotnet run --project tools/CodeIndex.Changelog -- check passed.
• git diff --check origin/main..HEAD passed.
• Repository dogfood status and workspace-manifest checks were healthy; the direct-deserialization audit found no remaining target-path call in DbReader.FilesStatus.cs.
• Mandatory Codex adversarial review completed with no blocking/actionable issues.

Documentation and changelog

• Updated the English and Japanese status-contract documentation in README.md and DEVELOPER_GUIDE.md.
• Updated the shared status contract in AGENT_GUIDE.md.
• Added changelog.d/unreleased/5161.fixed.md.

Follow-up candidates

None.

Fixes #5161