[1 of 3] Support long raw TUI goal objectives (#27508)

## Stack

1. **[1 of 3] Support long raw TUI goal objectives** - this PR
2. [2 of 3] Support long pasted text in TUI goals - #27509
3. [3 of 3] Support images in TUI goals - #27510

## Why

`thread/goal/set` limits persisted objective text to 4000 characters.
The TUI used to reject raw `/goal` objectives above that limit, even
though the client can make them usable by writing the long text to a
file and storing a short objective that points at that file.

This also needs to work for remote app-server sessions: filesystem API
calls must create files on the app-server host, and the stored path must
be meaningful to the agent on that host.

## What Changed

- Adds an app-server-host path helper so TUI code can build paths that
are resolved on the app-server host rather than the TUI host.
- Adds TUI app-server session helpers for `fs/createDirectory`,
`fs/writeFile`, `fs/readFile`, and `fs/remove` that work for embedded
and remote app-server sessions without changing the app-server protocol.
- Materializes oversized raw `/goal` objectives into
`$CODEX_HOME/attachments/<uuid>/goal-objective.md` through the
app-server filesystem APIs, then stores a short, readable objective that
directs the agent to that file.
- Reads managed objective files back for `/goal edit`. Other goal UI
renders the readable stored objective normally, without
managed-file-specific presentation logic.
- Recognizes managed references only when they name the expected
generated file under the app server's reported `$CODEX_HOME`, and cleans
up newly materialized files when goal replacement or setting does not
complete.

## Verification

- Added/updated TUI tests for raw oversized `/goal` submission, large
inline-paste expansion, queued oversized goals, app-facing
materialization before `thread/goal/set`, managed-path validation,
editing, and cleanup.
- Added/updated app-server-client remote coverage for initialized remote
Codex home handling.

## Manual Testing

- Ran the real TUI against a Unix-socket app server with different local
and server `$CODEX_HOME` directories. Oversized goals wrote only under
the server home, and persisted references used the server-canonical path
rather than the TUI path.
- Exercised 3,999-, 4,000-, and 4,001-character raw objectives. The
first two stayed inline without new files; the 4,001-character objective
became a managed objective file.
- Submitted a larger 8,275-character objective, verified its full
contents on the app-server host, and observed the goal continuation open
the referenced server-side file.
- Opened `/goal edit` for a managed objective and verified the full text
was restored through remote `fs/readFile`.
- Submitted an oversized replacement while a goal was active, verified
no file was written before confirmation, then canceled and confirmed
that the existing goal and attachment count were unchanged.

codex-rs/tui/src/app_server_session.rs

@@ -3,6 +3,8 @@
 //! This module owns the typed JSON-RPC calls needed by the TUI and keeps
 //! request/response plumbing out of `App` and `ChatWidget`.
 
+mod fs;
+
 use crate::bottom_pane::FeedbackAudience;
 use crate::legacy_core::config::Config;
 use crate::permission_compat::legacy_compatible_permission_profile;
@@ -14,6 +16,7 @@ use crate::status::plan_type_display_name;
 use crate::terminal_visualization_instructions::with_terminal_visualization_instructions;
 use codex_app_server_client::AppServerClient;
 use codex_app_server_client::AppServerEvent;
+use codex_app_server_client::AppServerPath;
 use codex_app_server_client::AppServerRequestHandle;
 use codex_app_server_client::TypedRequestError;
 use codex_app_server_protocol::Account;
@@ -247,6 +250,13 @@ impl AppServerSession {
         matches!(&self.client, AppServerClient::InProcess(_))
     }
 
+    pub(crate) fn codex_home_path(
+        &self,
+        local_codex_home: &AbsolutePathBuf,
+    ) -> Option<AppServerPath> {
+        self.client.codex_home(local_codex_home)
+    }
+
     pub(crate) fn server_version(&self) -> Option<&str> {
         let AppServerClient::Remote(client) = &self.client else {
             return None;