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).

AGENTS.md

@@ -40,7 +40,9 @@ repo is just where that cross-repo standard is authored and stored.
 - `retrofit-playbook.md` — terminal runbook for moving a project's brain onto disk.
 - `subagents-handbook.md` — designing and running delegated agents.
 - `guides/` — **neutral substance**: the self-contained operating guide for each command
-  and agent, written as plain prose any vendor's harness could follow.
+  and agent, written as plain prose any vendor's harness could follow. Also holds shared
+  reference docs that aren't tied to one command — e.g. `placement.md` (where a new project
+  should live), which `/new-project` and `how-i-work.md` both point at.
 - `adapters/claude/{commands,agents}/` — **thin Claude wrappers**: frontmatter + a pointer
   to the matching `guides/` file. Substance never lives in a wrapper.
 - `INBOX.md` — cross-project capture buffer for untriaged ideas/bugs (see below).
@@ -93,14 +95,24 @@ should carry this so any vendor's agent surfaces pending items at session start:
   diffable over time); latest snapshot dated 2026-06-14.
 - `/new-project` is the inverse of `/retrofit`: workshops a captured `(new)` idea into a
   standards-compliant repo and publishes to Gitea via a manual-create gate; the stack quality
-  gate is deferred to a future `/harden`.
+  gate is deferred to a future `/harden`. Now carries a **form-factor gate** (is this even a
+  standalone repo, or a feature/skill/agent of something that exists? — bail + reroute if so),
+  a **worth-building gate** at sign-off (build effort + ongoing tax → BUILD/PARK/ADOPT), a
+  **placement** step that walks `guides/placement.md`, the falsifiable-exit "substance rule,"
+  and a posture section — all harvested from a retired `idea-workshop` skill. A new project's
+  architectural decisions live in its `AGENTS.md` `## Decisions` section (no separate ADR file).
+- `guides/placement.md` is a new shared reference (where a project should run / which model /
+  what data layer), ported from that skill. **Its infra facts are UNVERIFIED** — generated
+  one-shot from chat history — and need a review pass with me (ROADMAP item 7); the file and
+  `how-i-work.md` both flag this.
 - Cross-repo git-hygiene audit done: 9 repos audited, no leaks, 6 remediated + pushed; the
   canonical `.claude/` block is now **deny-by-default** (in `portability.md`; full detail in
   git log + ROADMAP item 6).
 - Nothing in progress — all of this session's work is committed and pushed; tree clean.
-- **Next steps:** (1) the cross-repo quality-gate standard + `/harden` (ROADMAP item 1 — also
-  unblocks `/new-project`'s deferred quality-gate step); (2) the non-git-folder sweep under
-  `~/Projects` (item-6 residual).
+- **Next steps:** (1) verify & correct `guides/placement.md`'s infra facts with me (ROADMAP
+  item 7 — cheap, unblocks trustworthy placement in `/new-project`); (2) the cross-repo
+  quality-gate standard + `/harden` (ROADMAP item 1 — also unblocks `/new-project`'s deferred
+  quality-gate step); (3) the non-git-folder sweep under `~/Projects` (item-6 residual).
 - Queued elsewhere / specced-not-built: the `ten31-transcripts` mini-retrofit waits in
   `INBOX.md` for that repo's `/triage`; the SessionStart hook (item 3) and inbox-line bootstrap
   threading (item 4) remain on the ROADMAP.