fix(opencode): support opencode 2.x only - session_v2, debug paths, session delete, --standalone (Issue #60)

Measured on opencode 2.0.32 (macOS):
- the TUI rejects --agent ("Unrecognized flag"), so create and resume could
  not start opencode at all; the default agent is build. MAM now spawns
  `opencode --auto --standalone` (resume adds --session <id>): the server
  runs as the TUI's child and ends with it, and the per-user background
  service is neither used nor started;
- sessions live in session_v2 (a fresh 2.x database has no `session` table;
  a migrated one keeps the 1.x tables but no longer writes them). discover,
  verify_artifact, reconcile drift C and status read session_v2, and
  discover / drift C skip child (subagent) sessions;
- `db path` is gone (it prints the help and exits 0): the db comes from
  the `db` line of `opencode debug paths`, else the default path;
- `opencode session delete` removes a session with its child sessions from
  the 2.x tables (and leaves the 1.x ones): purge runs it with --standalone
  for a session in session_v2, still deletes the 1.x message/part/session
  rows, and residual also reports a session_v2 row left by a failed delete.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

.agents/skills/multi-agent-mux-resume/scripts/update_yaml_resumed.sh

@@ -238,10 +238,10 @@ elif agent == 'grok':
 elif agent == 'opencode':
     target['pane']['cmd'] = 'opencode'
     if uuid:
-        target['pane']['cmd_full'] = f'opencode --session {uuid} --auto --agent build'
+        target['pane']['cmd_full'] = f'opencode --session {uuid} --auto --standalone'
         target['opencode_session_id_own'] = uuid
     else:
-        target['pane']['cmd_full'] = os.environ.get('CMD_FULL_OPT', '') or 'opencode --auto --agent build'
+        target['pane']['cmd_full'] = os.environ.get('CMD_FULL_OPT', '') or 'opencode --auto --standalone'
     cp = os.environ.get('CHILD_PID', '0')
     if cp.isdigit() and int(cp) > 0:
         target['child_pid'] = int(cp)