Add Agent Fleet: a live grid of every agent pane

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Description

Running several agents at once means switching sessions to find out which one is
waiting for an answer. This adds a top-level view that shows them all at once,
grouped by project, status or agent.

• Grid of tiles, one per agent pane, with status accent, branch and age
• Cheap ANSI-stripped snapshots by default; a tile becomes a real xterm on hover,
  or all of them with "All live"
• Type into a focused tile without leaving the grid, optionally expanding it to
  the agent's full screen
• Fold projects away, jump to the next blocked agent (N), move with the arrow
  keys, close a pane
• Extracts the screen-text selection shared with the RunPane CLI into
  services/panels/terminalScreenText.ts

Two constraints are load-bearing and documented where they are enforced: a tile
renders at exactly the PTY's dimensions (agent TUIs use absolute cursor
positioning, so a mismatched width wraps every line and scrolls forever), and a
tile never answers the agent's terminal queries — the real panel is the single
voice on that PTY.

Type of Change

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING.md guidelines
• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• I have run pnpm typecheck and pnpm lint locally
• I have tested the Electron app locally with pnpm electron-dev
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation (AGENTS.md)

Critical Areas Modified

• State management/IPC events (two new fleet: channels, registered through
  the shared command registry and covered by daemonRegistryBindings.test.ts)

Screenshots (if applicable)

Additional Notes

Tests: fleetGrouping (14) covers grouping, ordering and pane labelling.

These four feature PRs are independent but all add an entry to the same
navigation plumbing (navigationStore's ActiveView, the two sidebar
components, preload.ts, api.ts, electron.d.ts). Whichever lands first,
the others need a small rebase there — no logic overlaps.

Not done: the packaged-build check from CONTRIBUTING. I develop on Windows,
so pnpm build:mac was not run.