Add metadata-only session search

[luoyuctl]

Summary

• add agenttrace --search for metadata-only local session lookup across names, paths, cwd, source/model, tools, tool args, files, anomalies, authority, and diagnostic evidence
• add text and JSON search reports without indexing prompt or assistant message text by default
• document the workflow and protect it with CI contract/doc smoke checks

Refs #101.

Verification

• go test ./...
• go vet ./...
• go build -o /tmp/agenttrace ./cmd/agenttrace
• git diff --check
• scripts/ci/check-output-contract.sh /tmp/agenttrace
• scripts/ci/check-report-semantics.sh /tmp/agenttrace
• scripts/ci/check-deterministic-output.sh /tmp/agenttrace
• scripts/ci/check-docs-commands.sh /tmp/agenttrace
• scripts/ci/check-release-surfaces.sh
• scripts/ci/check-pages-artifact.sh site

[luoyuctl]

Self-review: checked the PR diff, local validation, and CI. The feature is intentionally metadata-only: it searches session names, paths/cwd, source/model, tools/tool args, files, anomalies, authority, and diagnostic evidence, while avoiding prompt/assistant text indexing by default. #101 remains a broader radar item; this PR lands the narrow lookup workflow without adding a watcher or full search index.