## Why
The TUI still reached through `app-server-client::legacy_core` for
thread-name normalization and project-instruction filename details. In
particular, checking the TUI's local filesystem for `/init` is incorrect
for remote app-server sessions, where the server owns the working
directory and instruction discovery.
## What changed
- use the instruction source paths supplied by the app server to decide
whether `/init` should avoid overwriting project instructions
- keep the small thread-name normalization helper local to the TUI
- remove the now-unused instruction filename constants, utility module,
and other unused `legacy_core` re-exports
- make status helper tests independent of concrete instruction filenames
## Verification
- `just test -p codex-app-server-client`
- `just test -p codex-tui
slash_init_skips_when_project_instructions_are_loaded`
- `just test -p codex-tui` ran 2,799 tests; 2,797 passed and two
unrelated guardian feature-flag tests failed reproducibly in untouched
code
### Manual test
Started an app server over WebSocket with a remote workspace containing
`AGENTS.md`, then connected the TUI using `--remote`. After confirming
`thread/start` returned the file in `instructionSources`, deleted
`AGENTS.md` and ran `/init` in the existing session.
The TUI still reported that project instructions already existed and
skipped `/init`. The trace contained no `turn/start` request, confirming
the decision came from app-server session state rather than a new
client-local filesystem check.
This is a follow-up to https://github.com/openai/codex/pull/15922. That
previous PR deleted the old `tui` directory and left the new
`tui_app_server` directory in place. This PR renames `tui_app_server` to
`tui` and fixes up all references.
This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.
Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.