feat(editor): clearer edit controls — Edits label, Save emphasis, Save/Cancel

[backnotprop]

Polishes the markdown edit-mode controls (from #890) based on direct UX feedback. No server changes; plan-mode behavior is unchanged.

Changes

1. Annotation panel: "Direct edits" → "Edits"

The pinned direct-edits card label line-broke at narrow panel widths. Shortened to "Edits".

2. Save button — state-machine driven, zero layout shift

• Now reads the editableDocuments state machine (saveStatus) as the single source of truth, instead of a parallel editorDiffersFromBaseline flag. (The old logic was also inverted — it lit up when already saved.)
• Shows a dot + emphasis only when there are unsaved disk changes; muted "Saved" when in sync; "Saving" in flight.
• Destructive (red) dot + label on a failed save (disk conflict / write error) so it reads as "save failed, retry."
• No reflow: the dot slot is always reserved (only its color changes) and the label width is pinned to its widest value ("Saving") via an invisible sizer, so Save/Saving/Saved swap without nudging neighbors.

3. Edit exit control: Save + Cancel

• A source-backed session with unsaved edits now shows a two-step "Cancel" → "Discard?" that discards edits and exits — replacing the ambiguous "Done" that some users feared would lose their work.
• Plan mode and clean source sessions keep "Done" (commit edits + exit). Plan-mode keep behavior is intentionally unchanged, since plan mode has no disk-save and relies on commit-on-exit.

Behavior note: in source-backed mode the two-button model (Save = keep to disk, Cancel = discard) removes the old path of sending unsaved edits to the agent as a diff without writing to the file. Plan mode still supports that.

Test

• bun run typecheck — clean
• bun run build:hook — bundles clean
• Verified the Save state ladder (clean/dirty/saving/saved/conflict) and the Cancel two-step (arm → discard, reset on save/exit) by tracing the editableDocuments transitions; plan-mode path confirmed untouched.