feat: allow session-only delete on the main worktree

[xico42]

Summary

The dashboard hard-blocked d on the main worktree with a flat "cannot delete the main worktree", so the only way to kill that worktree's agent or shell sessions was to leave the TUI and run ch delete session by hand. On the CLI, ch delete worktree <project> main fell through to git, which failed removing the clone directory and surfaced a raw porcelain error.

This lets the TUI's d key kill the main worktree's sessions (leaving the worktree in place) and gives ch delete worktree main a clean refusal.

What changed

• Domain guard. The main worktree is the clone directory itself, not a checkout under __worktrees/, so tearing it down never makes sense. Teardown now refuses it with a new herd.ErrMainWorktree sentinel, checked the moment the clone dir is known — before any session is stopped or any git command runs.
• One error vocabulary. Both front ends translate the sentinel: the CLI names the worktree and points at ch delete session; the TUI shows a concise line.
• TUI. d no longer refuses the main worktree outright — it only declines when there are no sessions. With a session present it opens the confirm menu in a main-worktree mode that offers session kills only and never lists worktree removal, so the destructive option is unreachable by construction. A new deleteAllSessions action stops every session via StopOpts{All: true} and leaves the worktree in place.

Verification

make check green — 628 unit tests, integration tests, lint (0 issues), build; coverage 85.4% (≥ 80%). Includes a new integration test asserting Teardown refuses the main worktree for both --force values and leaves the clone dir on disk.

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