[ghcp-handoff] T6: Add worktree gc subcommand for merged branches

[bminier]

Problem

`ghcp-worktree.ts` has `add` and `remove` but no garbage collection. Over time `.gstack/worktrees/` accumulates directories for branches that are already merged to main.

Approach

`bun run bin/ghcp-worktree.ts gc`:

• List worktrees in `.gstack/worktrees/*`
• For each, check whether its branch is merged into the default branch OR whether its PR is closed/merged (via `gh`)
• Remove worktrees whose branches are safely mergeable

Dry-run by default; `--force` to actually remove.

Part of the G8 management story.

Review reference

Adversarial review T6

[bminier]

Fixed on branch ghcp-t6-worktree-gc (stacked on ghcp-t9-rabbit-holes-at-verify).

bun run bin/ghcp-worktree.ts gc [--dry-run] [--force] removes ghcp worktrees that are either (a) on branches that no longer resolve or (b) on branches fully merged to the repo's default branch. Preserves unmerged work; ignores worktrees outside .gstack/worktrees/.

Regression caught during test authoring: git branch --merged main prefixes branches checked out in other worktrees with + (not * , which marks the current branch). A parser that only stripped * would miss every ghcp branch since they all live in ghcp worktrees. Now strips both.

Pairs with prune: gc reclaims git state, prune reclaims artifact files. Different halves of "get this hard drive back."

Will close when merged to main.