## 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:
```text
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.
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