fix(#2069): forward model_policy, model_profile_overrides, runtime from global defaults (#2442)

* test(#2069): add fail-first regression for global-defaults dropped keys

Adds four failing-first regression cases to tests/defaults-json-fallback.test.cjs:

- model_policy forwarded from ~/.gsd/defaults.json
- model_profile_overrides forwarded from ~/.gsd/defaults.json
- runtime forwarded from ~/.gsd/defaults.json
- parity: model_policy survives identically whether it lives in the global
  defaults or in a project's .planning/config.json

All four fail on unfixed code (Branch D of loadConfigResolved builds
_globalBaseCfg from a whitelist that omits these three keys). The
project-config path at config-loader.cts:602-604 already forwards them,
so the global path should too.

* fix(#2069): forward model_policy, model_profile_overrides, runtime from global defaults

The _globalBaseCfg whitelist in Branch D of loadConfigResolved previously
omitted three keys that the project-config path forwards parsed['…']:

  - runtime
  - model_profile_overrides
  - model_policy

so ~/.gsd/defaults.json silently dropped them. A machine-wide model policy
(or runtime / profile overrides) was honored inside a project (where
.planning/config.json carries it) but ignored for out-of-project runs —
resolve-model fell back to the profile default with no warning.

Adds the three entries to _globalBaseCfg in the same (globalDefaults['…']) || null
shape as the sibling keys and the project-config path, so global defaults
honor them identically.

Regression tests in the prior commit (#2069 fail-first) demonstrate the
fix on the same suite that previously failed.

* test(#2069): extend parity test to all three previously-dropped keys

Code review (subagent) flagged that the parity test only asserted
model_policy shape-parity between global-defaults and project-config paths.
A future regression breaking just runtime or just model_profile_overrides
shape (e.g. someone changing parsed['runtime'] to ?? null in the project
path) would slip a single-key test.

Extends the parity test to assert deepStrictEqual / strictEqual across
all three keys: model_policy, model_profile_overrides, runtime. Same
two-dir setup, three cheap assertions.

* chore(changeset): backfill pr:2442 in .changeset/sturdy-seals-fly.md

src/config-loader.cts

@@ -731,6 +731,14 @@ function loadConfigResolved(cwd: string, options: Record<string, unknown> = {}):
         fast_mode: (globalDefaults['fast_mode']) || null,
         agent_skills: (globalDefaults['agent_skills']) || {},
         response_language: (globalDefaults['response_language']) || null,
+        // #2069: forward model_policy / model_profile_overrides / runtime so the global-defaults
+        // path is at parity with the project-config path (which forwards these three from
+        // parsed['…'] at the top of this function). Without these entries, ~/.gsd/defaults.json
+        // silently drops them — model_policy/provider/budget etc. are honored when set in a
+        // project but ignored when set globally.
+        runtime: (globalDefaults['runtime']) || null,
+        model_profile_overrides: (globalDefaults['model_profile_overrides']) || null,
+        model_policy: (globalDefaults['model_policy']) || null,
       };
       // Branch D: global-defaults
       try {