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>

brainstorm/SKILL.md

@@ -31,6 +31,12 @@ Triggers:
 **Skipping is permitted only** for:
 
 - A bug-fix iteration (use `debug` directly).
+- A test-specifiable feature, on a project whose profile
+  enables the `tdd` phase (use `tdd` directly — the RED
+  executable-spec is the spec, no prose spec needed). The
+  boundary is the design line: the moment one honest minimal
+  assertion cannot pin the behaviour without choosing between
+  plausible designs, `tdd` bounces back here.
 - A tidy iteration (use `audit` directly).
 - A trivial mechanical edit (per the project's CLAUDE.md
   "trivial mechanical edits" carve-out).
@@ -408,6 +414,12 @@ Hand off carries:
 
 - **Output target:** `../planner/SKILL.md` — only valid next
   skill.
+- **Alternative entry path / bounce-back source:**
+  `../tdd/SKILL.md` — on profiles that enable it, a
+  test-specifiable feature enters through `tdd` instead of
+  here; `tdd` bounces the work back to `brainstorm` the moment
+  the behaviour stops being test-specifiable (a genuine design
+  fork). Such a bounce-back arrives as a normal cycle request.
 - **Agent dispatched:** `agents/grounding-check.md` —
   dispatched in Step 7.5 as a hard-gate. The agent reads the
   spec with fresh context and reports PASS / BLOCK /