Adopt Pierre edit mode with session drafts

[danielss-dev]

Summary

• replace Strand's custom textarea/highlight overlay with Pierre's lazy-loaded <File edit> surface
• retain unsaved working-tree buffers in session memory across tab, pane, view, workspace, and repository navigation
• keep drafts aligned with Strand-initiated file moves and deletes
• add an explicit Discard changes action that reloads disk content without writing
• update the dependency, tests, task tracking, README, roadmap, learnings, and Work user guide

Why

Editing a file previously depended on Strand's custom textarea projection and the buffer was lost when the file document unmounted. Pierre now provides a native edit mode, and users need to move around the app without being forced to save incomplete work.

Impact

File writes remain explicit through Save or Mod+S, with the existing optimistic stale-write guard intact. Drafts live only for the current app session and are cleared after saving, discarding, deleting the file, or closing the repository. Historical revisions and unsupported file types remain read-only.

Validation

• pnpm --filter ./ui exec tsc --noEmit
• pnpm --filter strand-ui test — 53 files, 337 tests passed
• pnpm --filter strand-ui build
• cargo build --no-default-features
• live WebView2/CDP verification: draft survived navigation; discard cleared the draft and restored disk text; SHA-256 remained unchanged until an explicit save