fix(studio): deep-dive fixes — tool-error bodies, public reasoning, handler 500s, event log costs

[LucasWaki]

Summary

• Application error bodies ({"error": {...}} with status completed) now count as tool errors in attempt reports and stories; the story no longer claims every call succeeded.
• Public-audience reports keep the provider's reasoning summaries out of the story timeline.
• The HTTP handler answers unexpected defects with a 500 (exception name only, traceback to the log) instead of dropping the connection; the scheduler logs the exceptions it survives.
• Studio.emit caches the event count instead of re-counting the file per event; the SSE stream tails the log by byte offset.
• The CLI loop applies the Studio rule: no text or an abnormal stop reason is not a finished answer.
• run_story handles repetitions; two O(n²) trace scans are one-pass; duplicate who in usage.py, shadowed field, late-binding lambda and unused imports removed.
• Browser suite updated for the story-first attempt sheet and the new report section; the fixture server's process tree is killed on Windows.

Test plan

• uv run python -m pytest tests -q — 2128 passed, 3 skipped
• node tests/browser/suite.cjs — all checks pass, no stale listener left

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