feat(agents): enforce worktree-per-ticket in /implement's first step

[jinyeow]

Problem

AGENTS.d/git-worktrees.md documents the bare-worktree layout's core rule: a new branch means
a new worktree, never git checkout -b inside the default-branch worktree. The rule was
advisory only, so nothing stopped /implement from running directly on main.

Solution

/implement's own first step now detects the repo's default branch (never hardcoded, so it
still works on a repo whose default isn't main) and, if the current branch is the default
branch, branches off automatically before doing anything else: git worktree add in the
bare-worktree layout, git checkout -b in a normal clone. No hook, no marker file, no new
state, just plain skill-instruction text.

dispatch-implement's dispatch step now branches off once, before dispatching any child
(parallel or sequential), whenever the invoking session itself starts on the default branch.
Otherwise /implement's new check would spawn one throwaway worktree per ticket, and the
integration step would cherry-pick every child's commits back onto the default branch.

Full rationale and rejected alternatives (a PreToolUse hook + marker file, a hard-blocking
hook, confirm-before-create) are recorded in
docs/adr/implement-enforces-worktree-per-ticket-in-its-own-first-step.md.

Review

Reviewed with quick-review (Fable folded reviewer plus Codex on gpt-5.6-sol); all 7
findings raised were fixed: a bare-worktree-detection bug (git worktree list always shows
the current worktree, so the original check misclassified every normal clone), an
EnterWorktree scoping gap for pinned subagents, an explicit stop-and-ask fallback for Codex
CLI/Pi's unverified cd persistence, multi-ticket branch naming for dispatch-implement, and
three documentation-quality fixes.

Fixes #221