fix(agent-branch-start): verify worktree exists before printing Ready:

[NagyVikt]

Summary

git worktree add has been observed (twice in one Claude session on a downstream consumer of this template) to return exit code 0 with the target directory missing on disk. The script then ran OpenSpec / dependency-symlink init against the phantom path, eventually printing `[agent-branch-start] Ready:` and returning success. Callers cd into the vanished directory, claim file locks against it, and silently lose every subsequent edit until `gx branch finish` later complains about a missing tree.

Changes

Two defensive checks in templates/scripts/agent-branch-start.sh:

1. Immediately after git worktree add returns 0 — fail fast before the init phase runs against a phantom path.
2. At the top of print_agent_next_steps — covers the case where the dir disappears during init (rare, but cheap to verify).

Both report the branch name + expected path and instruct the operator to run `git worktree prune` before retry.

Test plan

• Shellcheck via `bash -n` passes.
• Run `gx branch start --tier T0 "test verify" "claude"` on a clean repo — verify normal happy path still works.
• Force the failure mode (e.g. `mkdir -p` race or manually removing the worktree dir between `worktree add` and the verify) — confirm exit 1 with the expected error message.

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