This PR completes the conversion of non-interactive `codex exec` to use
app server rather than directly using core events and methods.
### Summary
- move `codex-exec` off exec-owned `AuthManager` and `ThreadManager`
state
- route exec bootstrap, resume, and auth refresh through existing
app-server paths
- replace legacy `codex/event/*` decoding in exec with typed app-server
notification handling
- update human and JSONL exec output adapters to translate existing
app-server notifications only
- clean up "app server client" layer by eliminating support for legacy
notifications; this is no longer needed
- remove exposure of `authManager` and `threadManager` from "app server
client" layer
### Testing
- `exec` has pretty extensive unit and integration tests already, and
these all pass
- In addition, I asked Codex to put together a comprehensive manual set
of tests to cover all of the `codex exec` functionality (including
command-line options), and it successfully generated and ran these tests
This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.
Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.
Eric Traut