feat: enable GrowthBook local gate defaults for P0/P1 features

Add LOCAL_GATE_DEFAULTS mapping in growthbook.ts with 27 feature gate
defaults (25 boolean + 2 object config). Insert local defaults into the
fallback chain of all getter functions so they work regardless of whether
GrowthBook is enabled or disabled:

  env overrides → config overrides → in-memory cache → disk cache
  → LOCAL_GATE_DEFAULTS → caller defaultValue

P0 (local): keybindings, streaming tool exec, cron, JSON tools,
ultrathink, explore/plan agents, deep link, immediate model switch
P1 (API): session memory, auto memory, prompt suggestions, brief mode,
verification agent, away summary, auto dream, idle return prompt
Kill switches: 10 gates kept true to prevent remote disable

New compile flags: AGENT_TRIGGERS, ULTRATHINK, BUILTIN_EXPLORE_PLAN_AGENTS,
LODESTONE, EXTRACT_MEMORIES, VERIFICATION_AGENT, KAIROS_BRIEF, AWAY_SUMMARY

Bypass all local defaults: CLAUDE_CODE_DISABLE_LOCAL_GATES=1

build.ts

@@ -17,6 +17,16 @@ const DEFAULT_BUILD_FEATURES = [
   'SHOT_STATS',
   'PROMPT_CACHE_BREAK_DETECTION',
   'TOKEN_BUDGET',
+  // P0: local features
+  'AGENT_TRIGGERS',
+  'ULTRATHINK',
+  'BUILTIN_EXPLORE_PLAN_AGENTS',
+  'LODESTONE',
+  // P1: API-dependent features
+  'EXTRACT_MEMORIES',
+  'VERIFICATION_AGENT',
+  'KAIROS_BRIEF',
+  'AWAY_SUMMARY',
 ]
 
 // Collect FEATURE_* env vars → Bun.build features