feat(acp): add session store, session manager, and error handling

[im10furry]

Summary

Add ACP session infrastructure for managing in-memory and persistent sessions.

New Files

\src/acp/errors.ts\

• \AcpError\ class with JSON-RPC error codes and structured error data
• \ oJsonRpcError()\ for safe error type conversion (preserves data across \AcpError\ / \JsonRpcError\ chain)

\src/acp/sessionManager.ts\

• \AcpSessionManager\ generic session manager
• LRU eviction when exceeding \maxSessions\ (default 100)
• TTL-based expiration via \cleanupExpired()\
• Automatic MCP client lifecycle management (close on eviction/delete)
• \AbortController\ cleanup on session replacement/removal
• Time-injectable
  ow\ option for deterministic testing

\src/acp/sessionStore.ts\

• Atomic disk persistence with temp-file + rename pattern
• Per-project session directories (~/.kode//acp-sessions/)
• \cleanupExpiredAcpSessions()\ for TTL-based file cleanup
• Path sanitization for session ID and project directory

Tests

• \ ests/unit/acp-jsonrpc-validation.test.ts\ — payload budget rejection, signal listener cleanup, error mapping
• \ ests/unit/acp-session-store-manager.test.ts\ — atomic persistence, TTL cleanup, session eviction with MCP client close

Verification

• Prettier format check — pass
• ACP files typecheck — clean (no errors in new files)
• Existing main typecheck errors (29) are pre-existing, unrelated to this PR