[ghcp-handoff] T2: Auto-suffix target-branch on collision

[bminier]

Problem

Target-branch derivation has no uniqueness check. If `ghcp/fix-auth-bug` already exists, `ghcp-worktree.ts add` fails. User then has to pick a different name manually.

Approach

In `cmdAdd`: if the target branch exists AND the user didn't explicitly ask to reuse it, auto-suffix `-2`, `-3`, etc. until we find a free name. Print the final chosen name in the JSON result so the skill can use it.

Escape hatch: an `--exact` flag to fail instead of auto-suffix.

Review reference

Adversarial review T2

[bminier]

Fixed on branch ghcp-t2-branch-uniqueness (stacked on ghcp-t6-worktree-gc).

ghcp-worktree.ts add --branch X --base Y now auto-suffixes to X-2 / X-3 / ... on collision instead of failing. The JSON result gains auto_suffixed: true and desired_branch: "<original>" so the pre-flight summary can tell the user the rename happened.

--exact preserves the old fail-fast behavior when a user explicitly wants one specific branch name.

Collision check covers both the branch ref (refExists) AND the worktree path — a dangling worktree from a deleted branch won't silently cause a path collision.

Walk caps at 100 attempts before giving up (bailout in case something corrupts the branch namespace).

Will close when merged to main.