refactor: delete exec-server and move execve wrapper into shell-escalation (#12632)

## Why

We already plan to remove the shell-tool MCP path, and doing that
cleanup first makes the follow-on `shell-escalation` work much simpler.

This change removes the last remaining reason to keep
`codex-rs/exec-server` around by moving the `codex-execve-wrapper`
binary and shared shell test fixtures to the crates/tests that now own
that functionality.

## What Changed

### Delete `codex-rs/exec-server`

- Remove the `exec-server` crate, including the MCP server binary,
MCP-specific modules, and its test support/test suite
- Remove `exec-server` from the `codex-rs` workspace and update
`Cargo.lock`

### Move `codex-execve-wrapper` into `codex-rs/shell-escalation`

- Move the wrapper implementation into `shell-escalation`
(`src/unix/execve_wrapper.rs`)
- Add the `codex-execve-wrapper` binary entrypoint under
`shell-escalation/src/bin/`
- Update `shell-escalation` exports/module layout so the wrapper
entrypoint is hosted there
- Move the wrapper README content from `exec-server` to
`shell-escalation/README.md`

### Move shared shell test fixtures to `app-server`

- Move the DotSlash `bash`/`zsh` test fixtures from
`exec-server/tests/suite/` to `app-server/tests/suite/`
- Update `app-server` zsh-fork tests to reference the new fixture paths

### Keep `shell-tool-mcp` as a shell-assets package

- Update `.github/workflows/shell-tool-mcp.yml` packaging so the npm
artifact contains only patched Bash/Zsh payloads (no Rust binaries)
- Update `shell-tool-mcp/package.json`, `shell-tool-mcp/src/index.ts`,
and docs to reflect the shell-assets-only package shape
- `shell-tool-mcp-ci.yml` does not need changes because it is already
JS-only

## Verification

- `cargo shear`
- `cargo clippy -p codex-shell-escalation --tests`
- `just clippy`

codex-rs/shell-escalation/src/unix/execve_wrapper.rs

@@ -0,0 +1,25 @@
+//! Entrypoints for execve interception helper binaries.
+
+use clap::Parser;
+use tracing_subscriber::EnvFilter;
+
+#[derive(Parser)]
+pub struct ExecveWrapperCli {
+    file: String,
+
+    #[arg(trailing_var_arg = true)]
+    argv: Vec<String>,
+}
+
+#[tokio::main]
+pub async fn main_execve_wrapper() -> anyhow::Result<()> {
+    tracing_subscriber::fmt()
+        .with_env_filter(EnvFilter::from_default_env())
+        .with_writer(std::io::stderr)
+        .with_ansi(false)
+        .init();
+
+    let ExecveWrapperCli { file, argv } = ExecveWrapperCli::parse();
+    let exit_code = crate::run(file, argv).await?;
+    std::process::exit(exit_code);
+}