✨ 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).

package.json

@@ -49,6 +49,7 @@
         "fix": "npm run fix:oxlint && npm run fix:oxfmt",
         "fix:oxfmt": "oxfmt ${LEFTHOOK_FILES:-.}",
         "fix:oxlint": "oxlint --fix src",
+        "branch": "./scripts/branch.sh",
         "release": "./scripts/release.sh",
         "maintain": "npm run maintain:knip; npm run maintain:outdated",
         "maintain:knip": "knip --include dependencies,exports,files",