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.

README.md

@@ -1,13 +1,15 @@
 # agent-tools
 
-Two small Gitea-automation CLIs, shipped together in one RPM (`agent-tools`).
-Both act as the **`unkin-agent`** user by minting a scoped Gitea token from
-Vault, so automated PRs and comments are attributed to the agent — not to
-whoever happens to run the command.
+Small Gitea-automation CLIs, shipped together in one RPM (`agent-tools`). They
+act as the **`unkin-agent`** user by minting a scoped Gitea token from Vault, so
+automated PRs, comments and pushes are attributed to the agent — not to whoever
+happens to run the command.
 
 - **`agentpr`** — create pull requests and post PR comments as `unkin-agent`.
 - **`watchpr`** — poll one or more PRs and exit when one changes in a way worth
   acting on.
+- **`agentws`** — manage per-branch git worktrees for `unkin-agent`, cloning
+  into Ben's source checkout and isolating agent work under the XDG cache.
 
 ## How it gets a token
 
@@ -23,6 +25,9 @@ Everything is configured by environment variables, all with defaults:
 | `AGENT_APPROLE_ROLE_ID` | built-in default | AppRole role_id (overridable) |
 | `GITEA_URL` | `https://git.unkin.net` | Gitea base URL |
 | `AGENT_LOGIN` | `unkin-agent` | login whose comments `watchpr` ignores |
+| `AGENTWS_SRC_ROOT` | `~/src/prodenv` | source-of-truth checkout root (`agentws`) |
+| `AGENTWS_ROOT` | `~/.cache/agentws` | worktree root (`agentws`) |
+| `AGENTWS_OWNER` | `unkin` | Gitea org that owns the repos (`agentws`) |
 
 ## agentpr
 
@@ -67,10 +72,59 @@ watchpr --once --json unkin/argocd-apps#42
 On a meaningful change `watchpr` prints the reason and the PR's current state,
 then exits 0. Use `--json` for machine-readable output.
 
+## agentws
+
+`agentws` gives an agent an isolated git worktree per branch without disturbing
+Ben's shared checkouts. Repos are cloned into the **source root**
+(`~/src/prodenv/<repo>`) so branches created here are visible in the main
+checkout too; the worktrees themselves live under the **worktree root**
+(`~/.cache/agentws/<repo>__<branch>`).
+
+```bash
+# Clone unkin/argocd-apps into ~/src/prodenv if missing, then add a worktree for
+# a new branch off the remote default branch. Prints the worktree path.
+agentws new argocd-apps --branch benvin/my-change
+
+# Branch off a specific base instead of the remote default
+agentws new argocd-apps --branch benvin/hotfix --from release-1.2
+
+# List managed worktrees (repo, branch, path)
+agentws list
+
+# Remove a worktree (by path or branch); refreshes the source repo afterwards
+agentws rm benvin/my-change
+agentws rm ~/.cache/agentws/argocd-apps__benvin-my-change --delete-branch
+
+# Remove every managed worktree and prune each source repo
+agentws clean
+
+# Print a fresh unkin-agent Gitea token
+agentws token
+```
+
+### Auth / credential-helper design
+
+Gitea tokens minted from Vault are short-lived (~1h), so `agentws` never
+persists one in a remote URL or in git config. Instead it wires itself as an
+**ephemeral git credential helper**:
+
+- `agentws token` prints a fresh token to stdout (handy for scripts).
+- `agentws credential get` speaks the git credential protocol on stdin and, for
+  the configured Gitea host only, emits `username=unkin-agent` +
+  `password=<fresh token>`.
+
+`agentws new` sets this up per worktree without touching the shared checkout: it
+enables `extensions.worktreeConfig` on the repo once, then writes
+`user.name` / `user.email` and `credential.helper = !<agentws> credential` to
+the **per-worktree** config. Clone/fetch use the same helper via a transient
+`-c credential.helper=...`; the shared `origin` URL is left clean. On worktree
+removal `agentws` fetches in `~/src/prodenv/<repo>` so its default branch stays
+current.
+
 ## Build & package
 
 ```bash
-make build                 # -> dist/agentpr, dist/watchpr
+make build                 # -> dist/agentpr, dist/watchpr, dist/agentws
 make test                  # go test -race ./...
 make rpm                   # build + package dist/agent-tools-<version>-1.x86_64.rpm
 ```