docs(scripts): correct model-resolution header to four steps (#6879)

check-agent-model-declared's WHY-THIS-EXISTS header described model
resolution as three steps, omitting CLAUDE_CODE_SUBAGENT_MODEL, which
outranks the model argument, the frontmatter pin, and inheritance.
Corrects the header to the real four-step order (already written into
.claude/agents/os-dev.md by PR #6871) and adds the adjacent note that
an availableModels-blocked value falls back to the inherited model,
not to the frontmatter pin.

Comment-only change; no behaviour change. The gate still asserts only
that the model: slot is non-empty (#6803).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F8q5J1MQyocgtNspb15fSn

Author: claude

scripts/check-agent-model-declared.mjs

@@ -2,13 +2,22 @@
 // check-agent-model-declared — asserts that every agent definition under
 // .claude/agents/ declares a `model:` in its frontmatter (#6803).
 //
-// WHY THIS EXISTS. The Agent tool resolves a subagent's model in three steps: an
-// explicit `model` argument on the dispatch call → the agent definition's
-// frontmatter → **inherit from the parent session**. When the first two are absent
-// the third always applies, silently. That makes "what model does this role run
-// on" a property of whoever happened to dispatch it, at whatever moment their own
-// session was on, rather than a property of the role — and it changes with no
-// signal, mid-term, invisibly.
+// WHY THIS EXISTS. Claude Code resolves a subagent's model in four steps (verified
+// 2026-08-09 against the subagent documentation, "Claude Code resolves the
+// subagent's model in this order"): the `CLAUDE_CODE_SUBAGENT_MODEL` environment
+// variable, when set → an explicit `model` argument on the dispatch call → the
+// agent definition's frontmatter → **inherit from the parent session**. When the
+// first three are absent the last always applies, silently. That makes "what
+// model does this role run on" a property of whoever happened to dispatch it, at
+// whatever moment their own session was on, rather than a property of the role —
+// and it changes with no signal, mid-term, invisibly. (`CLAUDE_CODE_SUBAGENT_MODEL`
+// outranks every other step, including this gate's own pin — see
+// `.claude/agents/os-dev.md` for the full order and both adjacent traps.)
+//
+// A second, smaller nuance in that same order: a value blocked by the
+// organization's `availableModels` allowlist does NOT fall back to the frontmatter
+// pin below — it falls back to the INHERITED model, i.e. straight into the
+// silent-inheritance failure mode this gate exists to catch.
 //
 // That is measured, not theoretical, and it fails in a shape worth naming:
 //