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-create/scripts/create_session.sh

@@ -213,7 +213,7 @@ if [ -z "$CMD_FULL" ]; then
     agy)    CMD_FULL="${RESOLVED_BIN} --dangerously-skip-permissions" ;;
     hermes) CMD_FULL="${RESOLVED_BIN} --yolo --accept-hooks" ;;
     grok)   CMD_FULL="${RESOLVED_BIN} --permission-mode bypassPermissions" ;;
-    opencode) CMD_FULL="${RESOLVED_BIN} --auto --agent build" ;;
+    opencode) CMD_FULL="${RESOLVED_BIN} --auto --standalone" ;;
     codex)    CMD_FULL="${RESOLVED_BIN} --dangerously-bypass-approvals-and-sandbox" ;;
   esac
 fi