Fold idea-workshop into /new-project; add placement reference

Harvest the retired idea-workshop skill into the current new-project flow:
- form-factor gate (is this even a standalone repo, or a feature/skill/agent
  of something that exists? bail + reroute if so)
- worth-building gate at sign-off (build effort + ongoing tax -> BUILD/PARK/ADOPT)
- placement step that walks the new guides/placement.md
- falsifiable-exit substance rule and a posture section
- architectural decisions land in the new repo's AGENTS.md ## Decisions section,
  absorbing the old DECISIONS.md function (no separate ADR file)

Add guides/placement.md (ported from the skill) and point how-i-work.md at it.
Its infra facts are UNVERIFIED (one-shot from chat history) and flagged for a
review pass with me (ROADMAP item 7).

ROADMAP.md

@@ -130,3 +130,21 @@ allow-by-default and would have *un-ignored* `premier-gunner`'s password-bearing
   website-landing, Grand-Cayman-paddleboard). Each needs `git init` + retrofit, or an explicit
   "scratch, don't track" decision.
 - **`start-os`** is an external upstream (Start9Labs/start-os) — out of scope, no action.
+
+## 7. Verify & correct the placement guide
+
+**Why:** `guides/placement.md` was ported from a one-shot `idea-workshop` skill that was
+generated from chat history and **never verified** against the actual setup. Its *decision
+sequence* and *substance rule* are sound; its **infrastructure facts** (Start9 services,
+Spark/Qwen3 model lineup, embeddings/TTS, network/access, data-layer defaults) are
+provisional and almost certainly need correcting. The file carries an "UNVERIFIED" banner
+pointing here until that's done.
+
+**What it touches if wrong:** `/new-project` (Phase 2 walks it for placement) and
+`how-i-work.md` (one-line pointer), so stale facts propagate into every new repo's placement
+decision. Worth a focused pass.
+
+**To do:** walk the "Infrastructure facts" section with the user section by section — confirm
+or correct each (running Start9 services, the Spark gateway endpoint + Qwen3 details, TTS/
+embeddings, WireGuard/iOS access, data-layer defaults) — then drop the UNVERIFIED banner and
+update the "last generated" line to a real "last verified" date.