fix(branch ensure-feature): treat sibling-worktree checkout as success (#211)

[PolyphonyRequiem]

Problem

Under the parallel-fleet apex convention the apex spawns sibling
worktrees per item (polyphony-item-3043, polyphony-item-9001,
…) all backed by the same shared bare repo. When polyphony branch ensure-feature runs from one worktree and the target feature branch
is already checked out in a sibling, git checkout <branch> refuses
with exit 128:

fatal: 'feature/3043' is already used by worktree at 'C:/.../polyphony-item-3043'

GitClient.CheckoutAsync throws ExternalToolException, the
outer catch in EnsureFeature wraps it as CacheError, and
the workflow tips over — even though the verb's sole job (make sure
the branch exists) is already satisfied.

Fix

Per #211 Option 1 (idempotent success):

• Narrow try/catch around the CheckoutAsync call inside the
  localExisted branch.
• Match ExternalToolException.Stderr against
  is already used by worktree at '<path>' via a partial regex.
• On match: treat existence as satisfied, surface the sibling worktree
  path on a new WorktreePath envelope field, set
  Action = "exists_in_other_worktree", and keep the existing push
  step intact for !remoteExisted (git push operates on refs
  and is unaffected by which worktree owns the checkout).
• Unrelated checkout failures still propagate to the outer catch and
  exit CacheError unchanged.

Envelope shape

BranchEnsureFeatureResult.Action now includes
exists_in_other_worktree. WorktreePath is the new nullable
field, populated only on that path. Consumers that ignore it keep
working — fully additive.

Tests

New BranchCommandsEnsureFeatureTests (7 tests, all green):

• Local + remote both exist → checked_out, no push
• Local in sibling worktree, remote exists → exists_in_other_worktree + WorktreePath, no push
• Local in sibling worktree, remote absent → still pushes
• Local exists, generic checkout failure (e.g. dirty tree) → CacheError (regression guard)
• Remote-only → fetch + tracking checkout
• Neither exists → create + push
• Missing --branch → RoutingFailure

Validation

• dotnet test tests/Polyphony.Tests — 3836 passed, 0 failed
• Invoke-Pester tests/lint-jinja-resolver.Tests.ps1 — 30 passed
• tests/lint/fixtures/verb-output-schemas.json — surgical insert
  of the new worktree_path field per the
  hand-curated-fixture convention

Closes #211