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>

README.md

@@ -23,6 +23,7 @@ The pipeline skills, each with the agents it primarily dispatches:
 | `implement` | Plan exists | code + tests, uncommitted in working tree | Standard iteration path |
 | `audit` | Cycle closing OR baseline drift suspected | drift report + regression report | Mandatory at cycle close |
 | `debug` | Bug observed | RED test in working tree + cause analysis | Mandatory RED-first for any bug |
+| `tdd` | Test-specifiable feature / issue (alternative to `brainstorm`) | RED executable-spec in working tree → `implement` mini-mode | Opt-in entry path; bounces to `brainstorm` on a design fork |
 | `fieldtest` | Orchestrator-dispatched post-audit | example fixtures + friction spec | Per-cycle optional; milestone fieldtest is the closing gate for a surface-touching milestone |
 | `docwriter` | API surface stable across N cycles | rustdoc / docstring sweep | Optional |
 | `boss` | User types `/boss` | autonomous-orchestrator session — dispatches the other skills until done-state or bounce-back | User-invoked, never auto-dispatched |