feat(tdd): add executable-spec-first entry skill + tdd-author agent

A new opt-in alternative to the brainstorm -> planner design entry,
for work whose desired behaviour is test-specifiable. Instead of a
prose spec the user approves, `tdd` produces one minimal RED
executable-spec ("how it should work") and treats it like a bug to
be fixed: the GREEN side hands off to `implement` mini-mode, the
same two-stage RED-first handoff `debug` uses.

Skill + agent split mirrors debug/debugger: a thin SKILL.md owns
trigger, dispatch, the orchestrator-side loop, and handoff; the
agent (tdd-author) carries the authoring discipline. The agent
guards two failure modes specifically — guessing a design into a
test (a genuine design fork bounces back to `brainstorm` rather
than ratifying an un-chosen design), and pushing harder on a
headline test that won't go green (one iteration that can't reach
GREEN triggers a reactive decompose into a ladder of BLOCKER
sub-tests; two failed rounds bounce to `brainstorm`).

The boundary is the design line: `tdd` owns the work iff one honest
minimal assertion pins it; otherwise `brainstorm` reclaims it. This
is distinct from the per-task TDD the implementer already practices
inside `implement` — `tdd` authors the one headline spec above it.

Wires the phase into the docs it references:
- docs/profile-schema.md: opt-in `tdd` pipeline slot
- README.md: skill table row
- docs/pipeline.md: entry-path in the graph, phase description, skip rule
- templates/project-profile.yml: commented-out slot

install.sh picks up tdd/ automatically (top-level dir with SKILL.md).

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

docs/profile-schema.md

@@ -207,12 +207,18 @@ pipeline:
   debug:
     trigger: bug                  # observable misbehaviour
     red_first: true               # RED test before any fix
+  tdd:                            # opt-in: omit the key to disable the entry path
+    trigger: test_specifiable_feature  # behaviour expressible as one failing test
+    red_first: true               # RED executable-spec before any implementation
+    alt_to: brainstorm            # alternative design entry; bounces back on a design fork
   boss:
     user_invoked: true            # autonomous-orchestrator mode, /boss
 ```
 
 Phases not listed are disabled for the project. A project that
-does not want a `fieldtest` phase simply omits the key.
+does not want a `fieldtest` phase simply omits the key. `tdd` is
+opt-in the same way: a profile that omits it keeps
+`brainstorm → planner` as the only design entry path.
 
 ## Example: minimal profile