Fix suggest_improvement storage safety

[Widthdom]

Summary

• Sanitize suggest_improvement GitHub issue titles before submission.
• Redact common credential/token patterns before persisting or submitting suggestions.
• Validate the resolved .cdidx suggestion-store directory is writable before taking the store lock, and report cdidx_dir in tool payloads.
• Prune stale or over-limit local suggestions into .cdidx/suggestions-*.archive.jsonl, with config/env controls.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~GitHubIssueReporterTests|FullyQualifiedName~SuggestionStoreTests|FullyQualifiedName~CdidxConfigFileTests|FullyQualifiedName~McpServerTests.SuggestImprovement" -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --changed-between origin/main HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json remains stale only because the full workspace scan hit E013_INDEX_EXTRACTION_STALLED in SymbolExtractor.JavaScriptTypeScriptSupport.cs; changed files are matched after the diff index.

Documentation And Changelog

• USER_GUIDE.md
• changelog.d/unreleased/1614.fixed.md
• changelog.d/unreleased/1796.fixed.md
• changelog.d/unreleased/1921.fixed.md
• changelog.d/unreleased/1940.fixed.md

Review

• Codex adversarial review found two actionable issues.
• Fixed: duplicate prune paths now persist the pruned live store.
• Fixed: writability probing now uses a unique CreateNew temp file and preserves any existing .write_probe.

Follow-Up Candidates

• Full workspace indexing can stall with E013_INDEX_EXTRACTION_STALLED; tracked by Full index can stall on current repository after branch refresh #2717.

Fixes #1614
Fixes #1796
Fixes #1921
Fixes #1940