fix: label worktree rows by the branch they are for, not the folder name

[xico42]

Problem

The main worktree vanished from ch list worktree and the TUI dashboard whenever its clone dir had a non-default branch checked out. Each row was labelled with whatever HEAD happened to be on, so a geomonitor clone sitting on docs/rbac-epic showed up as docs/rbac-epic — main never appeared, and nothing distinguished the main worktree from the branch it was holding. The CLI compounded it by dropping the head-state entirely.

Fix

Separate addressing from display. Ref stays the worktree's identity — the folder-derived key every operation feeds back — while a new resolveDisplay decides the label from authoritative sources rather than reconstructing it from the folder name:

• git's live branch is the ground truth of what's checked out.
• The branch a worktree is for — its original — is the configured default_branch for the main clone dir, or the branch a running session recorded in @codeherd_branch for any other worktree.
• A row is shown diverged, <original> (on <live>), only when HEAD has actually left that original — so the main clone on a feature branch stays main (on docs/rbac-epic) and remains spottable.

A worktree with no known original (a non-main worktree with no session) is never treated as diverged: it shows exactly what git reports. This keeps the folder name out of the label — rendering the folder identity is what produced chore-cron-rework (on chore/restore-cron-rework) for a worktree simply sitting on its own branch whose directory name happens not to match it. The session is a refinement, never a dependency: the common case is a worktree with no session, and it resolves from git alone.

Both surfaces now render through one FormatBranchLabel, so the CLI listing and the TUI agree on how a diverged or detached worktree reads. Display resolution moves after the session join in List, because a non-main worktree's original branch lives on its session, not in the git worktree entry.

Tests

• Domain unit — the full settled table: main diverged/detached, non-main no-session (shows live branch, no hint), session-proven divergence, session-agrees, detached-with-session recovery.
• Formatter unit — FormatBranchLabel.
• TUI unit — no-session-live and session-divergence renders through buildItems → delegate.
• CLI integration — real git + tmux isolation: TestListWorktree_noSession_showsLiveBranch (primary path) and TestListWorktree_sessionDivergence_showsRecordedBranch (real shell session).

make check passes: 85.5% coverage, integration green, lint 0 issues, build OK.

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