Fixes#20792
## Why
`/goal`-first threads are valid resumable threads, but they can be
missing from `codex resume` and app recents because discovery depends on
metadata derived from a normal first user message.
PR #21489 attempted to fix this by using the goal objective as
`first_user_message`. Review feedback pointed out that
`first_user_message` does more than provide visible text today: it gates
listing, supplies preview text, and participates in deciding whether a
later title should surface as a distinct thread name. Reusing it for the
goal objective could leave a `/goal`-first thread with
`first_user_message=<goal>` and `title=<later prompt>`, even though the
goal should only provide the initial visible preview.
This PR follows that feedback by and keeps the `first_user_message` as
is but introduces a new `preview` field to separate concerns. The
`preview` field is populated from the first user message or the goal
objective. We can extend it in the future to include other sources.
## What Changed
- Added internal thread `preview` metadata in `codex-state`, including a
SQLite migration that backfills from `first_user_message` and from
existing `thread_goals` objectives when needed.
- Treated `ThreadGoalUpdated` as preview-bearing metadata so goal-first
threads can be listed and searched without mutating
`first_user_message`.
- Updated rollout listing, state queries, thread-store conversion, and
app-server mapping to use preview metadata while continuing to expose
the existing public `preview` field.
- Preserved title/name distinctness behavior around literal
`first_user_message`, so a later normal prompt after `/goal` does not
surface as a separate name just because the goal supplied the initial
preview.
- Preserved compatibility for older/internal metadata writes by deriving
preview from `first_user_message` when explicit preview metadata is
absent.
## Verification
- Manually verified that a thread that starts with a `/goal <objective>`
shows up in the resume picker.
Eric Traut