add agentws prune

Agents leave their managed worktrees behind, and `agentws rm` takes one path at
a time with no idea whether a branch's work is safely upstream, so clearing an
accumulation by hand risks destroying unmerged commits.

- classify every managed worktree: dirty, PR open, upstream, or unproven
- remove only what is safe; delete the local branch only when work is upstream
- prove "upstream" with merge-base and git cherry, so squash merges count
- match a PR by head.label, which survives the branch deletion a merge does
- dry run by default; --yes applies, --keep-branches spares every branch
- read the Gitea path from origin's URL rather than assuming the owner

cmd/agentws/main.go

@@ -13,6 +13,7 @@
 //	agentws new <repo> [--branch benvin/<name>] [--from <base-branch>]
 //	agentws list
 //	agentws rm <path-or-branch> [--delete-branch]
+//	agentws prune [--yes] [--keep-branches]
 //	agentws clean
 //	agentws token
 //	agentws credential get   # git credential-helper protocol on stdin
@@ -54,6 +55,7 @@ func newRootCmd() *cobra.Command {
 		newNewCmd(),
 		newListCmd(),
 		newRmCmd(),
+		newPruneCmd(),
 		newCleanCmd(),
 		newTokenCmd(),
 		newCredentialCmd(),