Fix search hints and exact substring highlights

[Widthdom]

Summary

• Add exact-substring recovery hints for punctuation-heavy search queries in CLI and MCP search responses.
• Expose literal-only highlight metadata for exact-substring compact JSON results.
• Add unreleased changelog fragments for both fixes.

Validation

• dotnet build
• dotnet test
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex adversarial review: No blocking/actionable issues found.

Issues

Fixes #2813
Fixes #2820

Follow-up issues

None.

Agent docs

AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.