Add bounded suggestion history queries

[Widthdom]

Summary

• Add redaction-safe NFKC full-text search across documented suggestion-history fields before pagination.
• Add authoritative count, capped summary, compact four-field, and UTF-8 byte-bounded JSON projections with truthful omission and recovery metadata.
• Keep sidecar reads bounded, preserve deterministic ordering, and fit whole rows with logarithmic prefix search.
• Document the new surface in the English and Japanese guides and add the required bilingual changelog fragment.

Root cause

Suggestion history exposed structured filters and pagination, but had no local text search or low-output aggregate/bounded projection. Duplicate preflight therefore required listing or exporting large portions of the local store.

User impact

Users can search stored suggestion history and answer duplicate-triage questions without exporting full descriptions or contexts. Pagination and byte truncation now expose authoritative counts, omission reasons, and a deterministic next offset while preserving existing redaction.

Validation

• Release solution build after rebasing onto the latest origin/main: 0 warnings, 0 errors.
• Final focused Release regression suite after adversarial-review fixes, CI manifest correction, and the latest rebase: net8.0 441 passed; net9.0 400 passed, 41 production-runtime tests skipped as designed.
• Full Release baseline: net8.0 11,237 passed / 7 skipped; net9.0 10,768 passed / 415 skipped.
• All JSON golden snapshots pass, including the new suggestions-compact.json contract.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore passed.
• The CI-failing CLI contract manifest test and all JSON snapshot tests pass on net8.0 and net9.0 (14/14 each).
• dotnet run --project tools/CodeIndex.Changelog -- check validated all 10 fragments.
• cdidx status --check reports the index matches the clean final workspace.
• Two required adversarial Codex review rounds completed; every reported finding was addressed, including logarithmic byte fitting, omission accounting, zero-length page recovery, testing-guide coverage, surrogate-safe title truncation, and golden snapshot coverage.
• A 5,000-record tight-byte-budget repro improved from about 9.37 seconds to 0.37 seconds while remaining valid bounded JSON.

Documentation and changelog

• Updated English and Japanese sections in USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5061.added.md with references to the completed foundational issues No CLI subcommand to list/inspect/export suggestions (suggestions invisible to users) #1876 and SuggestionStore.LoadAll has no filtering or pagination #1877.

Follow-ups

None.

Fixes #5061