docs: rule host built-ins out of the routing target universe

Closes the second open design decision on PR #103. The `/compact` finding was
recorded as a false positive needing an allowlist or a suppression mechanism.
It is neither: the routing universe is the apm marketplace, so a target either
resolves to a skill or an agent or it does not resolve, and `/compact`,
`/clear` and `/init` are Claude Code slash commands with no counterpart in
Copilot CLI or Codex. `.apm/` source compiles for all three, so a
vendor-neutral description routing to one is a portability defect and the hard
FAIL is a true positive.

An allowlist was rejected for a concrete reason, not a stylistic one: it
answers a different question ("does this exist on some host?"), it cannot
answer that portably from a single source file, and it goes stale the next
time a host ships a command — reintroducing the same-commit-two-verdicts
failure ADR-0020 already closed for deployed trees.

Nothing is blocked today: zero of the 43 descriptions name a host built-in,
and an author who needs to mention one writes it un-slashed, which is not
route notation and carries no routing claim.

Recorded in ADR-0020 and in both author-facing contract references, so the
next agent reads the decision rather than "fixing" the gate.

ADR: 0020

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/adr/0020-skill-description-and-body-context-contract.md

@@ -129,6 +129,16 @@ clause**, and a **boundary clause**. Capability enumeration, output-format detai
   dependency, and none needs to. Verified: a tree holding only `plugins/` and the root `apm.yml`,
   with no `.claude/` or `.agents/` anywhere, now produces findings identical to the working tree —
   26 description FAILs, 9 body FAILs, 2 dangling targets, 0 missing references, 58 SUGGESTIONs.
+- **The universe is the apm marketplace, and nothing else.** A routing target resolves to a skill or
+  an agent, or it does not resolve. Host built-ins are deliberately outside it: `/compact`, `/clear`
+  and `/init` are Claude Code slash commands with no counterpart in Copilot CLI or Codex, so a
+  vendor-neutral `.apm/` description routing to one is a portability defect and the hard FAIL is a
+  true positive, not a false one. An allowlist of known built-ins was **rejected**: it answers a
+  different question ("does this exist on *some* host?"), it cannot answer that portably from a
+  single source file, and it goes stale the next time a host ships a command — reintroducing the
+  same-commit-two-verdicts failure the bullet above exists to close. An author who needs to mention
+  one writes it un-slashed (``the `compact` built-in``), which is not route notation and makes no
+  routing claim.
 - **Blocking is scoped to a sentence, which makes sentence boundaries load-bearing.** A prose-form
   target earns a hard error only when its own sentence names another target that *resolves*; route
   notation (`/name`, `→ name`) is exempt and always blocks. So the splitter is part of the contract,

plugins/kyberforge/.apm/skills/agent-author/references/contract.md

@@ -70,6 +70,12 @@ that package declares in `apm.yml` under `dependencies.apm`. A sibling plugin in
 therefore resolves; a skill in an unrelated repo does not. A target outside that universe sends the
 router nowhere. Verify it before writing it — do not invent a plausible sibling.
 
+That universe is the apm marketplace and stops there. A **host built-in is not a routing target**:
+`/compact`, `/clear` and `/init` are Claude Code slash commands with no counterpart in Copilot CLI
+or Codex, and `.apm/` source compiles for all three, so routing to one is a portability defect. The
+gate is right to fail it and there is no allowlist. If a built-in genuinely needs mentioning, write
+it un-slashed — ``the `compact` built-in`` — which makes no routing claim and is not checked.
+
 **Length.** 250 characters SUGGESTION, 400 characters FAIL, counting the frontmatter value only
 with YAML folding resolved. Treat 250 as the target: the SUGGESTION tier is what moves the corpus
 average, the FAIL tier only stops outliers.

plugins/kyberforge/.apm/skills/skill-author/references/contract.md

@@ -57,6 +57,12 @@ therefore resolves; a skill in an unrelated repo does not. A boundary clause nam
 outside that universe sends the router nowhere and fails the audit. Check the target exists before
 writing it — do not invent a plausible sibling name.
 
+That universe is the apm marketplace and stops there. A **host built-in is not a routing target**:
+`/compact`, `/clear` and `/init` are Claude Code slash commands with no counterpart in Copilot CLI
+or Codex, and `.apm/` source compiles for all three, so routing to one is a portability defect. The
+gate is right to fail it and there is no allowlist. If a built-in genuinely needs mentioning, write
+it un-slashed — ``the `compact` built-in`` — which makes no routing claim and is not checked.
+
 **Length.** 250 characters SUGGESTION, 400 characters FAIL, counting the frontmatter value only
 with YAML folding resolved. The agentskills.io 1,024-character spec limit is unchanged and sits
 above both. The SUGGESTION tier is the one that moves the average; treat 250 as the target and 400

plugins/kyberforge/skills/agent-author/references/contract.md

@@ -70,6 +70,12 @@ that package declares in `apm.yml` under `dependencies.apm`. A sibling plugin in
 therefore resolves; a skill in an unrelated repo does not. A target outside that universe sends the
 router nowhere. Verify it before writing it — do not invent a plausible sibling.
 
+That universe is the apm marketplace and stops there. A **host built-in is not a routing target**:
+`/compact`, `/clear` and `/init` are Claude Code slash commands with no counterpart in Copilot CLI
+or Codex, and `.apm/` source compiles for all three, so routing to one is a portability defect. The
+gate is right to fail it and there is no allowlist. If a built-in genuinely needs mentioning, write
+it un-slashed — ``the `compact` built-in`` — which makes no routing claim and is not checked.
+
 **Length.** 250 characters SUGGESTION, 400 characters FAIL, counting the frontmatter value only
 with YAML folding resolved. Treat 250 as the target: the SUGGESTION tier is what moves the corpus
 average, the FAIL tier only stops outliers.

plugins/kyberforge/skills/skill-author/references/contract.md

@@ -57,6 +57,12 @@ therefore resolves; a skill in an unrelated repo does not. A boundary clause nam
 outside that universe sends the router nowhere and fails the audit. Check the target exists before
 writing it — do not invent a plausible sibling name.
 
+That universe is the apm marketplace and stops there. A **host built-in is not a routing target**:
+`/compact`, `/clear` and `/init` are Claude Code slash commands with no counterpart in Copilot CLI
+or Codex, and `.apm/` source compiles for all three, so routing to one is a portability defect. The
+gate is right to fail it and there is no allowlist. If a built-in genuinely needs mentioning, write
+it un-slashed — ``the `compact` built-in`` — which makes no routing claim and is not checked.
+
 **Length.** 250 characters SUGGESTION, 400 characters FAIL, counting the frontmatter value only
 with YAML folding resolved. The agentskills.io 1,024-character spec limit is unchanged and sits
 above both. The SUGGESTION tier is the one that moves the average; treat 250 as the target and 400