fix(#2041): map model_overrides full claude IDs to agent-tool aliases

model_overrides values that are full Claude model IDs (claude-sonnet-5,
claude-opus-4-8, claude-haiku-4-5, claude-fable-5) were returned verbatim on
the claude runtime and handed to the Claude Agent tool, whose typed model
parameter documents only tier aliases (opus/sonnet/haiku/fable). The
model_policy path already mapped full IDs -> aliases via
CLAUDE_POLICY_ID_TO_ALIAS (#1144); model_overrides skipped that mapping, so
the two resolver paths produced different shapes for the same underlying
Claude model. The fix mirrors #1144 on the override path via a shared
mapClaudeOverrideForRuntime helper used by both resolveModelInternal and
resolveModelForTier. Bare aliases pass through verbatim; non-Claude runtimes
and non-Claude custom/vendor values keep full IDs verbatim (parity). An
unmappable Claude ID (e.g. claude-opus-4-5) warns once to stderr and falls
through to tier resolution, exactly as the model_policy path already does.
Alias mapping is also the documented best practice (prevents staleness when
new model versions ship).

tests/helpers.cjs

@@ -423,6 +423,7 @@ function resetRuntimeWarningCaches() {
   const modelResolver = require('../gsd-core/bin/lib/model-resolver.cjs');
   configLoader._resetRuntimeWarningCacheForTests();
   modelResolver._resetModelPolicyWarningCacheForTests();
+  modelResolver._resetModelOverrideWarningCacheForTests();
 }
 
 module.exports = { runGsdTools, createTempDir, createTempProject, createTempGitProject, cleanup, parseFrontmatter, isUsageOutput, captureConsole, toPosixPath, runNpm, isolatedNpmEnv, withIsolatedProcessState, delay, waitFor, resetRuntimeWarningCaches, TOOLS_PATH };