Improve /goal error messages for ephemeral sessions

[etraut-openai]

Why

When a user runs /goal in a temporary session, the TUI can currently surface an internal app-server failure such as thread/goal/get failed in TUI. That message is technically true, but it does not explain the actual constraint: goals require a saved session because goal state is persisted with the thread.

This is especially confusing when codex doctor reports the background app-server as running in ephemeral mode, since that wording is easy to conflate with ephemeral thread/session behavior.

What changed

• Added a TUI-side formatter for thread-goal RPC failures in codex-rs/tui/src/app/thread_goal_actions.rs.
• Detects app-server/core errors that indicate goals are unsupported for an ephemeral thread/session.
• Replaces the internal RPC failure with a user-facing explanation:

Goals need a saved session. This session is temporary.
Run `codex` to start a saved session, or `codex resume` / `/resume` to reopen one.

• Preserves the existing generic failure wording for non-ephemeral goal errors.

Verification

• cargo test -p codex-tui thread_goal_error_message --lib

I also tried cargo test -p codex-tui; it built successfully but the test runner aborted in an unrelated side-thread stack overflow (app::tests::discard_side_thread_removes_agent_navigation_entry), which reproduced when run by itself.

[chatgpt-codex-connector]

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