Add agentws worktree-management binary

agentws manages per-branch git worktrees for the unkin-agent user: it clones
repos into the source root (~/src/prodenv/<repo>) so branches are visible in
Ben's main checkout, and creates isolated worktrees under the worktree root
(~/.cache/agentws/<repo>__<branch>).

- New internal/agent/git.go: small, testable git helpers shelling out to the
  git binary (clone/fetch/worktree add/remove/list/prune, branch + config ops,
  porcelain parsing, path sanitizing). No go-git dependency.
- New cmd/agentws: new / list / rm / clean / token / credential subcommands.
  Auth uses an ephemeral git credential helper (agentws credential get) so the
  ~1h Gitea token is never persisted in a remote URL or config; per-worktree
  config keeps the shared checkout's identity untouched.
- Wire agentws into Makefile, scripts/build-rpm.sh, packaging/nfpm.yaml (binary
  + bash/zsh/fish completions), .woodpecker/release.yaml (cross-compile + assets)
  and .gitignore.
- Tests: table tests for parsing/sanitizing/dir-naming, a real temp-git repo for
  the worktree lifecycle, and hermetic cmd tests (bad input + credential-helper
  host guard) that never touch the network.
- Document agentws in README.md and AGENTS.md.

.gitignore

@@ -1,7 +1,9 @@
 # built binaries (repo root only — not the cmd/ source dirs)
 /agentpr
 /watchpr
+/agentws
 # cross-compiled release artifacts (e.g. agentpr-linux-amd64)
 /agentpr-*
 /watchpr-*
+/agentws-*
 dist/