fix(session-ui): share timeline tool headers

[opencode-agent]

Summary

• Extract ToolHeader from BasicTool so ordinary timeline rows supply titles, details, arguments, paths, and actions instead of rebuilding header markup and typography.
• Reuse it for grouped Read/Grep/Glob/List rows, Shell/Execute, Webfetch, Edit/Write, reasoning, and tool-group summaries. Existing structured tool triggers use it automatically.
• Centralize 13px text with a 16px line height, consistent title/detail weights, and filename/directory truncation. Remove the separate Edit/Write header CSS and compact/group typography overrides.

Validation

• Production-component browser reproduction: baseline Write is 14px / 21px; shared header is 13px / 16px. The same font assertion fails on the baseline and passes on this branch.
• Added a deterministic production timeline story and browser coverage for typography, pending paths, light/dark narrow layouts, keyboard disclosure, and link semantics.
• bun typecheck in packages/session-ui and packages/app.
• Session UI unit tests: 166 passed.
• 47 scoped browser checks verified: 46 passed in the combined run; the pending-state test passed after correcting its locator, with no production-code changes.

Visual verification

• Before: e70d667a9fe3e84cc071a5596aa522c142c525b7; after: b530418425586cdaf2f3a7a074b9d3ebb98ab0eb.
• Real production timeline components in Storybook with identical deterministic tool data, dark theme, 1000×1000 viewport, device scale 1, and reduced motion.
• Both tool-group screenshots are 770×481. Intentional typography changes affect 3.194% of pixels; disclosure layout and row geometry remain unchanged.
• Separate tests cover 390px/1000px widths in light and dark themes.

Requested by: @Hona (Hona via Slack)