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/SKILL.md

@@ -35,8 +35,8 @@ orchestrator-agent every dispatch.
 Triggers:
 
 - A plan exists under `paths.plan_dir` (standard mode).
-- A `debug` skill has produced a RED test + cause and is
-  handing off for the GREEN side (mini mode).
+- A `debug` skill (RED test + cause) or a `tdd` skill (RED
+  executable-spec) has handed off for the GREEN side (mini mode).
 
 **Never skipped** when there is code to ship. Trivial
 mechanical edits (one-line typo fix, schema rename across N
@@ -91,15 +91,15 @@ Agent("implement-orchestrator", {
 })
 ```
 
-For a debug-handoff (mini mode):
+For a RED-first handoff from `debug` or `tdd` (mini mode):
 
 ```
 Agent("implement-orchestrator", {
   mode: "mini",
-  iter_id: "bugfix-<short-symptom>",
+  iter_id: "bugfix-<short-symptom>",   # tdd: "feat-<short-behaviour>"
   red_test_path: "<absolute path>",
-  cause_summary: "<1-2 sentences from debugger>",
-  constraint: "minimal fix, no surrounding cleanup"
+  cause_summary: "<1-2 sentences from debugger>",   # tdd: spec_summary, the desired-behaviour line
+  constraint: "minimal fix, no surrounding cleanup"   # tdd: "minimal feature, no surrounding scope"
 })
 ```