feat(web): add transcript export to the session menu

[dylantirandaz]

What this changes

• Adds an "Export transcript" section to the session context menu. It offers two formats: Markdown and JSON.
• The export contains the same transcript rows the screen shows, in the same order. A provenance header records the project, host, model, export time, and view freshness.
• The header warns when the export is partial: retained-history truncation, a cached or offline view, or a turn still running.
• Markdown caps long tool output at 4000 characters per block and says so. JSON carries the rows verbatim.
• Unknown entry kinds are written into the export, not dropped. The transient working indicator is omitted; the header covers it.

Implements the export row of FEATURE_MATRIX §3 (Markdown and JSON formats; HTML and a native save dialog are not included).

Verification

• pnpm check passes: release contract, provenance, lint, typecheck.
• pnpm --filter @t4-code/web test passes: 74 files, 1094 tests. The 15 new tests cover the header warnings, every row kind, the output cap, unknown-entry preservation, JSON fidelity, and filename rules.
• Opened the menu in a browser on fixture data and confirmed the wiring through React DevTools-level inspection: the export handler receives the current rows and runs both serializers. The download step uses the same Blob-anchor pattern as the existing activity export.

Checklist

• pnpm check passes (release contract, provenance, lint, typecheck)
• pnpm test passes; new behavior has a test that fails without this change (ran the @t4-code/web suite; the known packages/client transcript-retention failure on main is unrelated)
• Any pasted logs or screenshots are redacted: no secrets, tokens, pairing codes, or private paths
• Ported code (T3 Code / OMP) follows the provenance rules in THIRD_PARTY_NOTICES.md (no ported files touched)

[wolfiesch]

Made a few more quick changes:

• Kept the temporary working row out of JSON exports; active-turn state stays in the export metadata.
• Protected Markdown tool blocks when arguments or results contain backticks.
• Merged current main and reran pnpm check plus the full test suite.