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>

implement/agents/implement-orchestrator.md

@@ -125,7 +125,7 @@ ON `PARTIAL` OR `BLOCKED`, WRITE `BLOCKED.md` AT THE REPO ROOT BEFORE RETURNING.
   ```
   Make this RED test pass: <red_test_path>
 
-  Cause (from debugger):
+  Context (from debugger cause, or tdd spec_summary):
   <cause_summary>
 
   Constraint: <constraint>
@@ -258,8 +258,8 @@ E2E fixtures. Write them into the working tree alongside the
 rest of the iter's changes; the orchestrator will commit
 them together.
 
-(Mini mode: skip Phase 3 — the RED test from `debug` IS the
-coverage.)
+(Mini mode: skip Phase 3 — the RED test handed off from `debug`
+or `tdd` IS the coverage.)
 
 ### Phase 4 — On PARTIAL/BLOCKED, write BLOCKED.md