Workflow round 3: files pane, keyboard, project setup, notifications #1506

This follows #1493 and #1501.

H1. Files pane.

• Ship the declared but unshipped files pane (paneLayout.ts) as a worktree file tree with read-only and markdown preview.
• Syntax colour comes from feat(review): diff scopes, lazy per-file patches, split/wrap/whitespace toggles, syntax colour (#1493 C) #1500's highlighter.
• "Add to prompt" turns a selection into a composer chip.

H2. Keyboard.

• Remappable keybindings: one default table plus a JSON override, which also drives the keyboard sheet.
• A > actions-only filter in the palette.
• Back/forward thread history.
• ⌘Z undo for settle, snooze and archive.

H3. Project setup.

• A per-project "agent waits for setup" flag. waitForCommand exists but nothing calls it.
• Setup and quick actions read from a checked-in solenta.json.
• Submodule init in new worktrees.
• A configurable branch prefix.
• Clone from URL in Add project.

H4. Notifications and polish.

• A dock badge and an optional sound when threads wait for you.
• JetBrains editors, plus path:line:col in Open in.
• Mermaid diagrams in markdown.
• Em-dash cleanup in visible UI copy.