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-monitor/scripts/reconcile.sh

@@ -952,15 +952,20 @@ for s in d.get('herdr_sessions', []):
     try:
         conn = sqlite3.connect(odb)
         cursor = conn.cursor()
-        cursor.execute("PRAGMA table_info(session)")
+        # opencode 2.x keeps sessions in session_v2 (a migrated 1.x `session`
+        # table is no longer written).
+        cursor.execute("PRAGMA table_info(session_v2)")
         cols = [col[1] for col in cursor.fetchall()]
         if cols:
             cwd_col = "directory" if "directory" in cols else ("cwd" if "cwd" in cols else None)
             time_col = "time_created" if "time_created" in cols else ("created_at" if "created_at" in cols else ("started_at" if "started_at" in cols else None))
 
-            query = "SELECT id FROM session"
+            query = "SELECT id FROM session_v2"
             params = []
             where_clauses = []
+            if "parent_id" in cols:
+                # a subagent's session shares the directory and is newer
+                where_clauses.append("parent_id IS NULL")
             if cwd_col and cwd:
                 where_clauses.append(f"{cwd_col} = ?")
                 params.append(cwd)