docs: amend ADR-0020 and correct the gate reference to match what ships

The ADR said a /slash target behind a route verb takes the follower test; the
implementation decides notation first and skips it. Recorded as a dated amendment
rather than a silent edit, per the ADR-0016/0017 convention.

Its Enforcement table called itself exhaustive 'because the failure this ADR is
most exposed to is a rule filed under Enforcement that no validator implements'.
Three shipped behaviours were missing, including the disable-model-invocation
carve-out that removes two rows. Also de-pins the 'zoom-out is the one carrier'
claim, which caveman falsified, and the stale dangling-target statuses.

gates.md's ERROR row made terminality a conjunct for route notation, telling an
author a form is safe that exits 1. It now documents the references/ Vale blind
spot and its two independent causes -- AGENTS.md trimmed to the operative rule
per the split gates.md itself states -- plus the undocumented skill-frontmatter
hook and the second scope exclusion. CONTEXT.md glosses 'routing target'.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EJJrm5YmacbwMdzZpXcoti

CONTEXT.md

@@ -26,6 +26,14 @@ trigger clause, at most one capability clause, and a boundary clause, and nothin
 and the target-resolution walk: `docs/spec/gates.md`.
 _Avoid_: skill budget, size limit
 
+**Routing target**:
+The skill or agent name a boundary clause sends work to. It **resolves** when a skill or agent of
+that name is reachable from the file being checked, and **dangles** when none is — a route the router
+cannot take. Dangling is a blocking ERROR in route notation (`/name`, `→ name`) and a SUGGESTION for
+a bare name nothing else in the sentence corroborates. Verdicts and the resolution walk:
+`docs/spec/gates.md`.
+_Avoid_: route, pointer, cross-reference
+
 **Dispatch body**:
 The body pattern a skill with two or more mutually exclusive flows must use — the body carries only
 the dispatch table and the gates common to every branch, and each flow lives in its own