fix(#1133): honor model_policy on the claude runtime (forward-port to next) (#1144)

Forward-port of the #1133 hotfix (commit 22f237a5, 1.4.5 hotfix line) onto
next. The hotfix was authored against src/core.cts (v1.4.4); on next the
resolver logic lives in src/model-resolver.cts (ADR-457 extraction, #888),
so the patch is re-applied there rather than cherry-picked.

resolveModelInternal step 2.5 now honors model_policy on the claude runtime:
the policy-resolved full model ID is mapped back to a Claude Code agent alias
via CLAUDE_POLICY_ID_TO_ALIAS (reverse of MODEL_ALIAS_MAP + claude-fable-5 ->
fable). Bare aliases (opus/sonnet/haiku/fable) pass through; an ID with no
Claude alias warns once to stderr (deduped by agentType::policyModel::tier)
and falls back to the configured tier alias. Non-claude runtimes return full
IDs verbatim (unchanged). resolveModelForTier is intentionally unchanged.

The warn-dedupe cache lives in model-resolver.cts; core.cts composes the
exported _resetRuntimeWarningCacheForTests to clear both that cache and the
config-loader warning cache (config-loader cannot import model-resolver --
circular dependency).

Ports the 6 #1133 tests (rewriting the old claude-no-op test that asserted
the bug) plus one added test covering the MODEL_ALIAS_MAP reverse-map path.

Forward-port of #1133

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/nimble-eagles-fly.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1133
+---
+**`model_policy` is now honored on the default `claude` runtime** — including the `anthropic-fable` Claude Fable 5 preset. Policy-resolved model IDs map to Claude Code agent aliases (e.g. `claude-fable-5` → `fable`), and IDs without a Claude alias warn and fall back to the configured tier. Forward-port of #1133 (originally shipped on the 1.4.5 hotfix line). (#1133)