feat: add dry-run mode for hook execution (#2116) (#2188)
- Global --dry-run flag and ECC_DRY_RUN=1 env var
- Enriched preview: shows target file path, tool name, and command
- --dry-run stripped from argv so command routing works correctly
- Handles non-JSON and empty stdin gracefully (session/stop hooks)
- 10 tests covering isDryRun(), hook gating, enriched output, CLI routing
Naomi