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/prompts/prompt-manager.js

@@ -207,36 +207,4 @@ export async function loadPrompt(promptName, variables, config = null, pipelineT
     const promptError = handlePromptError(promptName, error);
     throw promptError.error;
   }
-}
-
-// Save prompt snapshot for successful agent runs only
-export async function savePromptSnapshot(sourceDir, agentName, promptContent) {
-  const snapshotDir = path.join(sourceDir, 'prompt-snapshots');
-  await fs.ensureDir(snapshotDir);
-
-  // Use deterministic naming - one snapshot per agent
-  const fileName = `${agentName}.md`;
-  const filePath = path.join(snapshotDir, fileName);
-
-  const timestamp = new Date().toISOString();
-  const snapshotContent = `# Prompt Snapshot: ${agentName}
-
-**Generated:** ${timestamp}
-**Agent:** ${agentName}
-
----
-
-## Full Interpolated Prompt
-
-\`\`\`markdown
-${promptContent}
-\`\`\`
-
----
-
-*This snapshot represents the exact prompt that was sent to Claude Code to generate the current deliverables for this agent.*
-`;
-
-  await fs.writeFile(filePath, snapshotContent);
-  console.log(chalk.gray(`    📸 Prompt snapshot saved: prompt-snapshots/${fileName}`));
 }