feat(workmux): short auto-name rules, gum write for the New prompt

docs/tools/workmux.md

@@ -21,6 +21,7 @@ workmux global config: one git worktree and one tmux session per agent, driven f
 | `worktree_dir` | `~/Work/{project}` | Worktrees land in `~/Work/<repo>/<handle>`, next to worktrunk's `~/Work/<repo>-<branch>`. |
 | `base_branch` | `auto` | New branches start from the repo's main branch, not the branch the pane is on. |
 | `agent` | `omp` | `<agent>` runs omp; it receives `-p`/`-P`/`-e` prompts and names branches for `add -A`. |
+| `auto_name.system_prompt` | rules below | Keeps `add -A` branch names short (see [Branch names](#branch-names)). |
 | `windows` | omp (focused), nvim | Two unnamed windows, so `automatic-rename-format` labels them like every other tab. |
 | `status_format` | `false` | workmux leaves `window-status-format` alone; `tmux.conf` renders `#{@workmux_status}` itself (see [Window tabs](../terminal/tmux.md#window-tabs)). |
 
@@ -29,3 +30,7 @@ A `.workmux.yaml` in a repo overrides any of these.
 ## Agent status
 
 The omp extension reports `working`, `waiting`, and `done`. workmux stores the icon in the window option `@workmux_status`, which the tab formats print after the label. The palette's Agents group reads `workmux status --all --json`.
+
+## Branch names
+
+`workmux add -A` (the palette's New item) pipes the task to `omp --no-session -p` with `auto_name.system_prompt`. The name becomes the worktree directory, the tab label (cut at 27 characters), and the session `<repo>/<name>` in the session pill (`status-left-length 30`), so the prompt asks for kebab-case, 1–2 words (3 if needed), at most 16 characters, the subject rather than the method, filler words dropped, common short forms (`auth`, `config`, `nav`, `anim`, …), and no `feat/`-style prefixes. Checked with `--dry-run`: a menu-bar tap/expand animation task → `menubar-expand`, a macOS codesigning CI failure → `codesign-fix`, plugin API docs → `plugin-api-docs`. The model can still overshoot; the prompt is guidance, not a hard cap.