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

implement/SKILL.md

@@ -210,7 +210,8 @@ Workflow({ name: "implement-loop", args: {
   mode: "standard",
   iter_id: "<iter_id>",
   plan_path: "<path under docs/plans>",
-  task_range: [3, 8]            // optional
+  task_range: [3, 8],           // optional
+  repo_root: "<abs path of the working tree>"  // strongly recommended; effectively required in worktree sessions (issue #25)
 }})
 ```
 
@@ -222,10 +223,19 @@ Workflow({ name: "implement-loop", args: {
   iter_id: "bugfix-<short-symptom>",   // tdd: "feat-<short-behaviour>"
   red_test_path: "<absolute path>",
   cause_summary: "<1-2 sentences from debugger>",   // tdd: the desired-behaviour line
-  constraint: "minimal fix, no surrounding cleanup"  // tdd: "minimal feature, no surrounding scope"
+  constraint: "minimal fix, no surrounding cleanup",  // tdd: "minimal feature, no surrounding scope"
+  repo_root: "<abs path of the working tree>"  // strongly recommended; effectively required in worktree sessions (issue #25)
 }})
 ```
 
+`repo_root` is stamped into every stage prompt as a `git -C` anchor:
+in worktree sessions a dispatched agent's cwd can resolve to the
+primary checkout and review the wrong tree (false `infra_blocked`
+quality reviews, a false no-op `BLOCKED` from the end-verify — issue
+#25). Without the field the first dispatched stage probes
+`git rev-parse --show-toplevel` as a fallback, which is subject to the
+same race — pass the field.
+
 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