feat(agent-manager): registry mirrors live agents

[codeaholicguy]

Summary

• Make AgentRegistry the source of truth for active agents: AgentManager.listAgents() is now the single writer, batching a registry entry for every detected agent and pruning dead pids in the same call.
• Persist sessionId + sessionFilePath per entry so cache-using adapters can skip their per-call session discovery walk.
• Codex and Gemini adapters short-circuit detection by looking up live pids in the registry; Claude and OpenCode keep their existing O(1) lookups (PID file / SQLite) but still get one registry entry written per live process.

Behavior

• One entry per live agent across all adapters after every listAgents() call.
• User-set names (e.g. "merry"), tmuxSession, and startedAt are preserved across cycles. Other fields refresh from the latest detection.
• tmuxSession merge rule on upsert: preserve existing non-empty when incoming is empty (auto-upserts from adapters never clobber a value set by agent start or future tmux integration).
• Pid recycle within the same agent type and same cwd is rare and deferred — accepted trade-off.