feat(conventions): substantive work isolates into git worktrees, merge user-ratified

Introduce the worktree convention the corpus previously lacked: any
substantive work stream targeting main — in every mode, autonomous
(/boss) and interactive alike — runs in a git worktree on its own
branch (issue-N-<slug>), and the branch reaches main only through a
user-ratified merge performed by the orchestrator (fast-forward
preferred, push covered by the ratification, no pull request).
Background sessions were already harness-isolated; this extends the
same isolation to the sessions the harness does not guard. The
convention depends on worktree.baseRef=head (the default 'fresh'
bases branches on origin's routinely-stale tip under the
no-autonomous-push discipline), now pinned in conventions.md.

boss/SKILL.md gains the run-level mechanics (§ The run worktree):
one worktree per run named after the reference issue, entered before
the first dispatch; the merge is a user-reserved run-close act; the
done-state notify names the branch as the review handle; closes #N
fires at merge-and-push time. Iron Law, rollback-sandbox framing,
user-reserved-act enumerations, rationalisations, and red flags
updated to match.

Corollary reconciliations after adversarial review: implement's
'there is no branch' narrowed to 'the loop creates no branch of its
own'; specify's auto-signed-commits-on-main heading reworded to
ratified-history framing; the sandbox bullets in conventions.md and
the CLAUDE.md fragment note the anchor is the run branch's base.

The skills meta-repo itself stays exempt (its CLAUDE.md: install
symlinks resolve to the primary clone) and direct-on-main.

implement/SKILL.md

@@ -219,7 +219,9 @@ Workflow({ name: "implement-loop", args: {
 The carrier fields are defined authoritatively at the top of
 `workflows/implement-loop.js`. The load-bearing `iter_id` rule is there
 too: it names the scratch dir and the stats filename, NOT a branch
-(there is no branch).
+(the loop creates no branch of its own; it runs in whatever checkout
+the session sits in — under the worktree convention, the run's
+worktree branch).
 
 ### Step 2 — Read the end-report