Support cross-environment thread discovery and messaging

[lastobelus]

Problem

PR #52 added first-class Codex thread inspection, send, and wait commands within one LastCode environment. A Codex thread running in one environment still cannot discover or address a thread owned by another environment.

The concrete use case is a small personal setup with two machines: Codex on Airy should be able to find, inspect, message, and wait for a thread running on htulo, and vice versa. Today this falls back to ad hoc SSH and direct remote SQLite inspection.

Desired outcome

• Discover configured LastCode environments and their reachable threads.
• Resolve a thread by explicit environment plus thread ID, with a convenient unambiguous lookup path for user-directed queries.
• Support the existing read, send, and wait operations across environments.
• Reuse authenticated LastCode environment transport and scoped credentials rather than reading a remote database directly.
• Preserve environment identity in command output so local and remote results cannot be confused.
• Keep the design appropriate for a single user with two machines and usually fewer than ten concurrent threads.

Example user-directed flows:

• What is THREAD_ID up to?
• Ask THREAD_ID some question.
• Tell THREAD_ID to do something.
• Do this, and when finished tell THREAD_ID to do that.

Scope constraints

• Tracking only; do not implement yet.
• Optimize for the existing two-environment personal deployment, not a general fleet or multi-tenant message bus.
• User-initiated workflows are sufficient initially; autonomous agent-created workflow graphs can wait.
• Avoid broad retry, reconciliation, or routing machinery unless a concrete failure in this deployment requires it.
• MCP may expose the capability, but should not force an unnecessary bridge if LastCode can directly serve the same authenticated environment operations.

Design questions for later

• Should remote environment discovery come from configured LastCode connections, MCP configuration, or a small explicit host registry?
• Should the CLI extend lastcode-thread with an environment selector, or add a separate environment-aware entry point?
• Can existing remote-link authentication issue the same read and operate scopes used by PR feat(lastcode): add Codex thread tools #52?
• What is the smallest addressing syntax that remains unambiguous when thread IDs or prefixes overlap across environments?

Dependency

Build on the local-environment primitives merged in PR #52. This issue intentionally defers implementation until we choose the transport and addressing model.