Fix Kiro post-February storage discovery

[ozymandiashh]

Summary

Fixes #329 by teaching the Kiro provider to discover the post-February 2026 extensionless storage layout while preserving the legacy .chat parser.

What changed

• Discover legacy .chat files, top-level extensionless session index files, and one-level nested extensionless execution files under each Kiro workspace hash directory.
• Skip { executions: [...] } session index files during parsing because they do not contain conversation content.
• Parse modern execution JSON for execution/session IDs, timestamps, model IDs, conversation text, direct prompt/response fields, structured tool calls, token estimates, and cost estimates.
• Preserve executions that contain structured tool calls without assistant text, while avoiding false tool names from top-level execution metadata.
• Skip hidden extensionless files during discovery.
• Add regression coverage for modern execution parsing, session-index skipping, extensionless discovery, and structured tool-only execution shapes.
• Update Kiro provider docs and changelog.

Closes #329.

Validation

• npx vitest run tests/providers/kiro.test.ts tests/provider-registry.test.ts — 36/36 tests passed.
• npm run build — passed.
• git diff --check — passed.
• Claude Opus 4.7 effort max review — PASS, no required fixes.
• Gemini 3.1 Pro Preview review — PASS, no required fixes.

Notes

Validation uses synthetic fixture shapes matching the public issue report. I did not use or include real local Kiro history, local project names, prompts, paths, or usage details.