[codex] Use app/list for TUI app catalog

[canvrno-oai]

Summary

This code cleanup moves TUI app catalog loading onto the backend app/list API so the TUI no longer assembles accessible and directory apps itself. It also wires app/list/updated notifications into the TUI cache and keeps the existing behavior around startup readiness, thread cwd config, and local enable-disable races.

• Request app lists through app-server from the TUI background request path.
• Let the backend own merged app data and enabled state for /apps and $ mentions.
• Preserve active thread cwd when evaluating app policy for app/list.
• Send a backend follow-up update when Codex Apps is still finishing startup.
• Update existing popup tests for backend-sourced app state and notification flow.

Testing

• just fmt
• git diff --check
• cargo test -p codex-tui
• cargo test -p codex-app-server

[canvrno-oai]

@codex review

[chatgpt-codex-connector]

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