feat: implement unified audit system v3.0 with crash-safety and self-healing

## Unified Audit System (v3.0)
- Implemented crash-safe, append-only logging to audit-logs/{hostname}_{sessionId}/
- Added session.json with comprehensive metrics (timing, cost, attempts)
- Agent execution logs with turn-by-turn detail
- Prompt snapshots saved to audit-logs/.../prompts/{agent}.md
- SessionMutex prevents race conditions during parallel execution
- Self-healing reconciliation before every CLI command

## Session Metadata Standardization
- Fixed critical bug: standardized on 'id' field (not 'sessionId') throughout codebase
- Updated: shannon.mjs (recon, report), src/phases/pre-recon.js
- Added validation in AuditSession to fail fast on incorrect field usage
- JavaScript shorthand syntax was causing wrong field names

## Schema Improvements
- session.json: Added cost_usd per phase, removed redundant final_cost_usd
- Renamed 'percentage' -> 'duration_percentage' for clarity
- Simplified agent metrics to single total_cost_usd field
- Removed unused validation object from schema

## Legacy System Removal
- Removed savePromptSnapshot() - prompts now only saved by audit system
- Removed target repo pollution (prompt-snapshots/ no longer created)
- Single source of truth: audit-logs/{hostname}_{sessionId}/prompts/

## Export Script Simplification
- Removed JSON export mode (session.json already exists)
- CSV-only export with clean columns: agent, phase, status, attempts, duration_ms, cost_usd
- Tested on real session data

## Documentation
- Updated CLAUDE.md with audit system architecture
- Added .gitignore entry for audit-logs/

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/phases/pre-recon.js

@@ -99,7 +99,7 @@ async function runPreReconWave1(webUrl, sourceDir, variables, config, pipelineTe
         AGENTS['pre-recon'].displayName,
         'pre-recon',  // Agent name for snapshot creation
         chalk.cyan,
-        { webUrl, sessionId }  // Session metadata for logging
+        { id: sessionId, webUrl }  // Session metadata for audit logging (STANDARD: use 'id' field)
       )
     );
     const [codeAnalysis] = await Promise.all(operations);
@@ -123,7 +123,7 @@ async function runPreReconWave1(webUrl, sourceDir, variables, config, pipelineTe
         AGENTS['pre-recon'].displayName,
         'pre-recon',  // Agent name for snapshot creation
         chalk.cyan,
-        { webUrl, sessionId }  // Session metadata for logging
+        { id: sessionId, webUrl }  // Session metadata for audit logging (STANDARD: use 'id' field)
       )
     );
   }