refactor(#888): extract model+effort resolution into model-resolver.cts (#890)

ADR-857 rollout phase 2f — the FINAL core.cts decomposition. Move the model and
effort resolution cluster (resolveModelInternal, resolveModelPolicy,
resolveTierEntry, _resolveRuntimeTier, resolveModelForTier,
resolveGranularityInternal, assertValidGranularityOverride, resolveEffortInternal,
resolveFastModeInternal, resolveEffortForTier, nextEffort + VALID_GRANULARITIES/
VALID_EFFORTS/EFFORT_SET + interfaces) out of core.cts into a new leaf module
src/model-resolver.cts. core.cts re-exports the 13 public symbols (callers in
init/docs/commands unchanged; export= set byte-identical).

Cycle-free: model-resolver imports only leaves (config-loader for loadConfig,
configuration for defaults, model-profiles + model-catalog for the static
tables). Removed 6 now-unused imports from core (verified zero remaining
references, none re-exported).

This completes the god-module decomposition: core.cts 2271 -> 389 lines (~83%),
now a thin re-export spine over seven clean leaves (io, phase-id, roadmap-parser,
core-utils, phase-locator, config-loader, model-resolver).

New-CLI-module checklist done (.gitignore, eslint, INVENTORY 96->97 + row,
manifest, ARCHITECTURE, CONTEXT.md "Model Resolver Module"). Adds
tests/model-resolver.test.cjs (81 tests: behavioral + shim-identity + adversarial).

Gates: lint, code-review (export set byte-identical; import-removal verified),
security-review, codex adversarial-review (all 0 findings; verbatim move). Mac
4303 pass; clean-build docker 13117 pass, 0 fail.

Closes #888

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

eslint.config.mjs

@@ -73,6 +73,7 @@ export default tseslint.config(
       'gsd-core/bin/lib/command-aliases.cjs',
       'gsd-core/bin/lib/config-schema.cjs',
       'gsd-core/bin/lib/model-profiles.cjs',
+      'gsd-core/bin/lib/model-resolver.cjs',
       'gsd-core/bin/lib/installer-migrations/002-codex-legacy-hooks-json.cjs',
       'gsd-core/bin/lib/installer-migrations/003-rename-get-shit-done-to-gsd-core.cjs',
       'gsd-core/bin/lib/observability/logger.cjs',