fix(app): group edit and write diffs with patches

[opencode-agent]

Summary

• Group adjacent edit, write, and patch calls into a single ordered file group, both directly in the timeline and inside “Used…” groups.
• Preserve file disclosure state as groups grow or split; failed tools and intervening content remain boundaries.
• Retain actual post-format write diffs in completed tool metadata, including overwrites, without changing machine output or model-facing confirmation text.
• Keep input-based rendering for older results without diff metadata, pending edits, and empty/unchanged edit or write results.

This brings the grouping work from #44977 onto current v2 without its header-removal changes.

Validation

• Core edit/write/patch tests: 66 passing.
• Session UI unit tests: 179 passing.
• Typechecks in packages/core, packages/session-ui, and packages/app.
• 47 focused Storybook browser regressions passed (patch-group.spec.ts, session-timeline.spec.ts, and file-tool.spec.ts), covering homogeneous/mixed tools, append/split disclosure state, failure/content boundaries, missing/zero-change metadata, pointer/keyboard disclosure, and narrow layouts.
• Real production app with identical seeded edit/write/patch results: grouped and separate placement, repeated paths, accordion click/keyboard toggling, and 420px layout. A separate narrow stress check covered 48 edit/write calls across 24 long paths, long diff values, wrapping, expansion, and scrolling without page overflow.
• Production file-group benchmark: five scenarios, three repetitions each, both before and after. All 30 runs passed; these are small shared-host samples, not a performance claim.

Existing failures outside this change

An additional Storybook sweep found five failures that also reproduce on the untouched base f16acb1c95ad4e289b466d3148f490b7070ba09a: four stale group-label expectations in session-tool-projection.spec.ts / tool-disclosure.spec.ts, and the existing textVisible(...).get(...) is not a function fixture error in the collapsed-stack story. These tests and their unrelated behavior are left out of scope; the 47 focused checks above pass.

Requested by: @Brendonovich (Brendan via Slack)