fix(implement-loop): stamp a repo-root anchor into every stage prompt

In worktree sessions a dispatched agent's cwd can resolve to the
primary checkout instead of the loop's working tree — observed as a
per-dispatch race: 6/9 quality dispatches returning infra_blocked with
the sha256-of-empty fingerprint over a real diff, and a tree-check
counting the primary checkout's clean tree into a false no-op BLOCKED
on a genuinely-DONE iteration.

Remedy (validated in the field on the reporting run): the carrier
gains repo_root, stamped into every stage prompt as an explicit
'git -C <root>' anchor with a wrong-directory tripwire (an empty diff
or unexpected HEAD sha = wrong cwd, re-run against the anchor). When
the carrier omits it, the first dispatched stage (mini preflight /
standard plan-index) probes 'git rev-parse --show-toplevel' as a
fallback — itself subject to the same race, so SKILL.md documents the
field as effectively required in worktree sessions.
compiler-driven-edit gets the same carrier field (same defect class,
carrier-only). agent-template stage list updated in passing
(plan-extract[-all], end-verify).

closes #25

docs/agent-template.md

@@ -159,8 +159,8 @@ pin exists to remove. Effort follows the model split:
 
 Workflow scripts pin a third tier inline: schema-bound
 extraction/verification stages that author no code (preflight,
-plan-index, plan-extract, snapshot, mini-verify, tree-check, finalize,
-build/suite verify) run `medium` via the `agent()` call's `effort:` option. As with
+plan-index, plan-extract[-all], snapshot, mini-verify, the standard
+end-verify, build/suite verify) run `medium` via the `agent()` call's `effort:` option. As with
 `model:`, every `agent()` call passes `effort:` explicitly — whether
 frontmatter effort propagates through an `agentType` dispatch is
 undocumented, so the scripts do not rely on it (see the policy header