✨ Add branch front-door command

The branching model assumed a clean, current `main` and a green baseline
before any edit, but both were prose, and prose nobody checks silently
becomes a suggestion. `npm run branch -- <prefix>/<desc>` asserts the
precondition and only then creates the branch, so a later failure is
attributable to the change that caused it.

Checks run cheap-first (`--porcelain` deliberately catches untracked
files, which would otherwise ride onto the new branch) and the suite
runs last, so an ineligible tree never pays for it. `main`'s remote is
derived from its upstream rather than hardcoded: this repo has both
`origin` and `origin_https`, and `main` tracks the latter, so a
`git fetch origin main` currency check would compare against a ref that
is never updated here. The baseline runs after switching to `main`, so a
red `main` restores the starting branch instead of stranding the caller
on it.

The prefix stays a judgment call: the script validates it against the
documented vocabulary instead of inferring it.

Prose kept as index only — AGENTS.md points agents at the command from
the task workflow, CONTRIBUTING.md owns the model and the bare-script
tier (dropping its stale hardcoded count of "two" conveniences).

AGENTS.md

@@ -41,7 +41,7 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 
 ### Working on tasks
 
-- **Task with subtasks** (a task that has indented children): create a branch off `main` using the prefix inferred from the task content (`feature/…` / `fix/…` / `chore/…`), check it out, work on each subtask with commits, then present a concise handover for the user to review. Use this fixed shape:
+- **Task with subtasks** (a task that has indented children): create the branch with `npm run branch -- <prefix>/<desc>`, inferring the prefix from the task content (`feature/…` / `fix/…` / `chore/…`) — do not hand-write `git switch -c`, the command enforces the clean-tree / current-`main` / green-baseline precondition. Work on each subtask with commits, then present a concise handover for the user to review. Use this fixed shape:
 
     ```md
     ## Handover — <branch>