feat(#160): wire claw list-sessions and delete-session CLI commands

Closes the last #160 gap: claws can now manage session lifecycle entirely
through the CLI without filesystem hacks.

New commands:
- claw list-sessions [--directory DIR] [--output-format text|json]
  Enumerates stored session IDs. JSON mode emits {sessions, count}.
  Missing/empty directories return empty list (exit 0), not an error.

- claw delete-session SESSION_ID [--directory DIR] [--output-format text|json]
  Idempotent: not-found is exit 0 with status='not_found' (no raise).
  Partial-failure: exit 1 with typed JSON error envelope:
    {session_id, deleted: false, error: {kind, message, retryable}}
  The 'session_delete_failed' kind is retryable=true so orchestrators
  know to retry vs escalate.

Public API surface extended in src/__init__.py:
- list_sessions, session_exists, delete_session
- SessionNotFoundError, SessionDeleteError

Tests added (tests/test_porting_workspace.py):
- test_list_sessions_cli_runs: text + json modes against tempdir
- test_delete_session_cli_idempotent: first call deleted=true,
  second call deleted=false (exit 0, status=not_found)
- test_delete_session_cli_partial_failure_exit_1: permission error
  surfaces as exit 1 + typed JSON error with retryable=true

All 43 tests pass. The session storage abstraction chapter is closed:
- storage layer decoupled from claw code (#160 initial impl)
- delete contract hardened + caller-audited (#160 hardening pass)
- CLI wired with idempotency preserved at exit-code boundary (this commit)

src/__init__.py

@@ -5,7 +5,16 @@ from .parity_audit import ParityAuditResult, run_parity_audit
 from .port_manifest import PortManifest, build_port_manifest
 from .query_engine import QueryEnginePort, TurnResult
 from .runtime import PortRuntime, RuntimeSession
-from .session_store import StoredSession, load_session, save_session
+from .session_store import (
+    SessionDeleteError,
+    SessionNotFoundError,
+    StoredSession,
+    delete_session,
+    list_sessions,
+    load_session,
+    save_session,
+    session_exists,
+)
 from .system_init import build_system_init_message
 from .tools import PORTED_TOOLS, build_tool_backlog
 
@@ -15,6 +24,8 @@ __all__ = [
     'PortRuntime',
     'QueryEnginePort',
     'RuntimeSession',
+    'SessionDeleteError',
+    'SessionNotFoundError',
     'StoredSession',
     'TurnResult',
     'PORTED_COMMANDS',
@@ -23,7 +34,10 @@ __all__ = [
     'build_port_manifest',
     'build_system_init_message',
     'build_tool_backlog',
+    'delete_session',
+    'list_sessions',
     'load_session',
     'run_parity_audit',
     'save_session',
+    'session_exists',
 ]