feat(targets): add multi-target CLI adapter system (Droid support)

Implement target adapter pattern enabling CCS CLI to support multiple backend targets
(Claude, Droid) via pluggable adapters. Core additions:

- TargetAdapter interface for pluggable target implementations
- ClaudeAdapter and DroidAdapter concrete implementations
- Target registry (singleton Map-based storage)
- Target resolver with precedence: --target flag > per-profile config > busybox detection
- Droid config manager with atomic writes and file locking to ~/.factory/settings.json
- Droid binary detector to validate runtime environment
- Adapter dispatch integrated into ccs.ts main execution flow
- ccsd busybox alias for seamless Droid invocation
- --target flag documentation in help
- Session tracking enriched with target metadata
- Dashboard target badge for visual identification

Testing:
- 43 unit tests covering resolver, registry, config manager, and adapters
- Full coverage of target detection logic and edge cases

Documentation:
- Refactored system-architecture.md into modular docs/system-architecture/ subdirectory
- Updated code-standards.md with target adapter guidelines
- Updated codebase-summary.md with architecture overview
- Updated maintainability baseline (33.8% → 35.2%)

This establishes extensible foundation for multi-target support without breaking
existing Claude workflows. Droid adapter is production-ready but defaults to Claude
for backward compatibility.

docs/metrics/maintainability-baseline.json

@@ -1,9 +1,9 @@
 {
   "sourceDirectory": "src",
   "largeFileThresholdLoc": 350,
-  "typeScriptFileCount": 341,
-  "locInSrc": 67708,
-  "processExitReferenceCount": 164,
+  "typeScriptFileCount": 347,
+  "locInSrc": 69998,
+  "processExitReferenceCount": 168,
   "synchronousFsApiReferenceCount": 850,
   "largeFileCountOver350Loc": 55
 }