Document scaffolding convention; drop frontmatter the hub does not use

No template repo: fork-at-create cannot reach existing repos, and the
scaffolding is three markdown files. Records what every component repo carries
instead, and the frontmatter rule inferred from orin-nano-env-setup — living
documents carry it, historical reports do not, the hub carries none.

ORIN-RUNTIME.md had frontmatter no other hub document uses. Removed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013kj58Jugkic783b8ZrtTVZ

docs/ORIN-RUNTIME.md

@@ -1,6 +1,3 @@
----
-type: default
----
 # Orin runtime
 
 [ARCHITECTURE.md](ARCHITECTURE.md) describes services and the bus without saying

docs/REPOS.md

@@ -110,3 +110,26 @@ docs/
 The `impl/plans` ÷ `impl/reports` split is worth keeping: plans go stale,
 reports do not. A report like `PCAN-SETUP.md` stays valuable precisely because
 it records the investigation and the dead ends, not just the final commands.
+
+### Scaffolding conventions
+
+No template repository for this. Template instantiation is fork-at-create, so it
+could not reach the repos that already exist, and the scaffolding is three
+markdown files — lighter than the machinery to manage it. A template earns its
+place where there is a real toolchain to carry, which is the ESP32 case above.
+
+What every component repo has instead:
+
+```
+README.md                 frontmatter, the shared blurb, "What this repo is
+                          about", then links to guidelines and impl/
+docs/GUIDELINES.md        frontmatter, numbered sections, ends "## N. To be added"
+docs/impl/plans/          at least INITIAL.md
+docs/impl/reports/
+docs/BOM.md               only where the repo owns hardware
+```
+
+**Frontmatter follows the document's nature, not its location:** living documents
+carry `type: default`; historical records do not. So README, GUIDELINES, BOM and
+plans have it, and reports do not. The hub carries none at all, being the portal
+rather than a component.