fix(ui/agents): drop Codex-incompatible --model example from custom args tab (#1310)

* fix(agent/codex): route custom_args -m/--model to thread/start payload

Codex agents spawn via `codex app-server --listen stdio://`, which does
not accept `-m` / `--model` (those belong to the normal Codex CLI). When
a user's custom_args still carried those tokens the process exited
before the JSON-RPC initialize handshake with `codex process exited`,
with no actionable error.

Extract `-m <v>`, `--model <v>`, and `--model=<v>` from opts.CustomArgs
before invoking app-server and promote the value into opts.Model, so
that startOrResumeThread can pass it through the `thread/start` payload
where Codex actually reads the field.

Fixes #1308.

* fix(ui/agents): drop Codex-incompatible --model example from custom args tab

The helper text and placeholder suggested `--model claude-sonnet-4-…` as
a custom CLI argument, which is valid for Claude but crashes Codex
agents (its `app-server` subcommand does not accept model flags). Swap
in provider-agnostic copy so the UI no longer steers users into an
invalid configuration for non-Claude runtimes.

Refs #1308.

* revert "fix(agent/codex): route custom_args -m/--model to thread/start payload"

This reverts f18355b2. After review, extracting `-m`/`--model` out of
opts.CustomArgs and promoting them into the thread/start payload is the
wrong shape of fix: agent CLIs have many flags their non-interactive
modes don't accept, and hand-translating a subset case-by-case doesn't
scale — it pushes us toward an ever-growing list of per-backend arg
rewriters.

The preferred direction is to teach users via the UI what command their
custom_args extend (see the launch_header preview in #1312) and let
bad configurations fail loudly. If the resulting error is hard to read
that's a separate improvement we should make on the failure path, not
by silently rewriting user input.

Refs #1308.

packages/views/agents/components/tabs/custom-args-tab.tsx

@@ -81,8 +81,8 @@ export function CustomArgsTab({
             Custom Arguments
           </Label>
           <p className="text-xs text-muted-foreground mt-0.5">
-            Additional CLI arguments appended to the agent command at launch
-            (e.g. --model claude-sonnet-4-20250514)
+            Additional CLI arguments appended to the agent command at launch.
+            Supported flags depend on the agent's CLI.
           </p>
           {launchHeader && (
             <p className="mt-2 text-xs text-muted-foreground">
@@ -111,7 +111,7 @@ export function CustomArgsTab({
               <Input
                 value={entry.value}
                 onChange={(e) => updateEntry(index, e.target.value)}
-                placeholder="--model claude-sonnet-4-20250514"
+                placeholder="--flag value"
                 className="flex-1 font-mono text-xs"
               />
               <button