fix(sdk): preserve nested keys from globalDefaults in configNewProject (#2745)

When building nested config sections (workflow, git, hooks, agent_skills,
features), the deep merge was missing globalDefaults for those sections,
causing user values from ~/.gsd/defaults.json to be silently dropped.

Added globalDefaults spread at the correct precedence level (hardcoded <
globalDefaults < userChoices) for all five nested keys, and added three
test cases verifying the merge works end-to-end via HOME env var override.

Closes #2673

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

sdk/src/query/config-mutation.ts

@@ -406,22 +406,27 @@ export const configNewProject: QueryHandler = async (args, projectDir, workstrea
     ...userChoices,
     git: {
       ...(defaults.git as Record<string, unknown>),
+      ...((globalDefaults.git as Record<string, unknown>) || {}),
       ...((userChoices.git as Record<string, unknown>) || {}),
     },
     workflow: {
       ...(defaults.workflow as Record<string, unknown>),
+      ...((globalDefaults.workflow as Record<string, unknown>) || {}),
       ...((userChoices.workflow as Record<string, unknown>) || {}),
     },
     hooks: {
       ...(defaults.hooks as Record<string, unknown>),
+      ...((globalDefaults.hooks as Record<string, unknown>) || {}),
       ...((userChoices.hooks as Record<string, unknown>) || {}),
     },
     agent_skills: {
       ...((defaults.agent_skills as Record<string, unknown>) || {}),
+      ...((globalDefaults.agent_skills as Record<string, unknown>) || {}),
       ...((userChoices.agent_skills as Record<string, unknown>) || {}),
     },
     features: {
       ...((defaults.features as Record<string, unknown>) || {}),
+      ...((globalDefaults.features as Record<string, unknown>) || {}),
       ...((userChoices.features as Record<string, unknown>) || {}),
     },
   };