fix: disable semantic inject by default — experimental feature not ready for all users

The per-prompt Chroma vector search injection on UserPromptSubmit adds latency
and context noise. Disable by default while we iterate on a more precise
file-context approach. Users can still opt in via CLAUDE_MEM_SEMANTIC_INJECT=true.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/SettingsDefaultsManager.ts

@@ -121,7 +121,7 @@ export class SettingsDefaultsManager {
     CLAUDE_MEM_EXCLUDED_PROJECTS: '',  // Comma-separated glob patterns for excluded project paths
     CLAUDE_MEM_FOLDER_MD_EXCLUDE: '[]',  // JSON array of folder paths to exclude from CLAUDE.md generation
     // Semantic Context Injection (per-prompt via Chroma vector search)
-    CLAUDE_MEM_SEMANTIC_INJECT: 'true',              // Inject relevant past observations on every UserPromptSubmit
+    CLAUDE_MEM_SEMANTIC_INJECT: 'false',             // Inject relevant past observations on every UserPromptSubmit (experimental, disabled by default)
     CLAUDE_MEM_SEMANTIC_INJECT_LIMIT: '5',           // Top-N most relevant observations to inject per prompt
     // Tier Routing (model selection by queue complexity)
     CLAUDE_MEM_TIER_ROUTING_ENABLED: 'true',         // Route observations to models by complexity