feat: add conflict-safe file editing

[wolfiesch]

Summary

• add lazy host-backed file browsing and previews to the Files pane
• edit only complete text previews and save through revision-gated files.write commands
• preserve drafts and surface conflicts when host content changes, a revision goes stale, the connection drops, or a save outcome cannot be confirmed

Verification

• pnpm check
• pnpm test
• Browser smoke: browsed the fixture tree, previewed package.json, and entered editor mode

Screenshots

[Copilot]

Pull request overview

Adds conflict-safe, revision-gated file editing to the Files pane, integrating controller-backed saves and draft/conflict handling so the UI can safely edit only complete previews and avoid overwriting host changes.

Changes:

• Introduces per-path file drafts (clean/dirty/saving/conflict/error) with save/discard actions and conflict detection on preview refresh.
• Adds files.write action availability and wires live + fixture controllers to dispatch revision-gated writes and resolve outcomes.
• Extends tests to cover local draft lifecycle, revision-stale conflicts, and save result settling.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
apps/web/test/panes-live.test.ts	Extends live-pane tests for files.write enablement and revision-gated save/conflict behavior.
apps/web/test/panes-files.test.ts	Adds unit tests for draft editing, save promotion, conflict on host change, and truncated-preview edit blocking.
apps/web/src/features/panes/model.ts	Adds fileWrite to the action availability model.
apps/web/src/features/panes/live-inspector.ts	Wires files.write availability and implements revision-gated writeFile controller behavior.
apps/web/src/features/panes/inspector-store.ts	Adds draft state/actions and implements preview/write outcome reconciliation logic.
apps/web/src/features/panes/fixtures.ts	Extends fixture controller to persist edited content and resolve write outcomes as saved.
apps/web/src/features/panes/FilesPane.tsx	Adds editor UI (textarea) and Save/Discard controls driven by draft and fileWrite availability.

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