docs(tdd): wire sibling skills to the new entry path

The tdd skill referenced its neighbours (implement, brainstorm,
debug) but none referenced it back. Close the loop so the new
executable-spec-first entry path is reachable and consistent from
every skill that describes a relationship it now belongs to:

- implement: mini-mode trigger + dispatch example now cover a
  RED-first handoff from `debug` OR `tdd` (was debug-only); the
  orchestrator's task template and Phase-3 skip note generalised.
  This was real drift — mini-mode is no longer debug-exclusive.
- planner: skip rule gains the `tdd` case (it skips brainstorm
  AND planner — the RED executable-spec is the plan).
- brainstorm: `tdd` added to the permitted-skip list as the
  profile-gated alternative entry path, plus a cross-ref marking
  brainstorm as the bounce-back target when behaviour stops being
  test-specifiable.
- boss: pipeline diagram, Step-3 routing prose, and cross-refs.
  A test-specifiable feature issue is dispatched to `tdd`
  autonomously, the same way a bug issue goes to `debug`; this is
  NOT a new-cycle bounce-back (the test is the spec). The
  bounce-back fires only reactively, when tdd surfaces a genuine
  design fork.
- debug: reciprocal sibling note + cross-ref (new behaviour is
  tdd's job; debug is for regressions of existing behaviour).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

debug/SKILL.md

@@ -40,6 +40,12 @@ Trigger this skill on:
 change, not a fix. Ad-hoc judgement that a bug is "too small
 for TDD" is the exact failure mode this skill exists to prevent.
 
+`debug` is for a regression of *existing* behaviour. New
+test-specifiable behaviour is its sibling `tdd`'s job (same
+two-stage RED→GREEN shape, triggered by a feature description
+rather than an observed misbehaviour) — route there instead,
+on profiles that enable it.
+
 ## The Iron Law
 
 ```
@@ -93,3 +99,7 @@ separate one.
   GREEN side after the orchestrator has decided whether to
   commit the RED test separately or as part of the final fix
   commit.
+- **Sibling RED-first skill:** `../tdd/SKILL.md` — same
+  two-stage RED→GREEN handoff to `implement` mini-mode, but
+  triggered by a new test-specifiable behaviour rather than an
+  observed bug.