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/app-server/tests/suite/zsh

@@ -0,0 +1,72 @@
+#!/usr/bin/env dotslash
+
+// This is the patched zsh fork built by
+// `.github/workflows/shell-tool-mcp.yml` for the shell-tool-mcp package.
+// Fetching the prebuilt version via DotSlash makes it easier to write
+// integration tests that exercise the zsh fork behavior in app-server tests.
+//
+// TODO(mbolin): Currently, we use a .tgz artifact that includes binaries for
+// multiple platforms, but we could save a bit of space by making arch-specific
+// artifacts available in the GitHub releases and referencing those here.
+{
+  "name": "codex-zsh",
+  "platforms": {
+    // macOS 13 builds (and therefore x86_64) were dropped in
+    // https://github.com/openai/codex/pull/7295, so we only provide an
+    // Apple Silicon build for now.
+    "macos-aarch64": {
+      "size": 53771483,
+      "hash": "blake3",
+      "digest": "ff664f63f5e1fa62762c9aff0aafa66cf196faf9b157f98ec98f59c152fc7bd3",
+      "format": "tar.gz",
+      "path": "package/vendor/aarch64-apple-darwin/zsh/macos-15/zsh",
+      "providers": [
+        {
+          "url": "https://github.com/openai/codex/releases/download/rust-v0.104.0/codex-shell-tool-mcp-npm-0.104.0.tgz"
+        },
+        {
+          "type": "github-release",
+          "repo": "openai/codex",
+          "tag": "rust-v0.104.0",
+          "name": "codex-shell-tool-mcp-npm-0.104.0.tgz"
+        }
+      ]
+    },
+    "linux-x86_64": {
+      "size": 53771483,
+      "hash": "blake3",
+      "digest": "ff664f63f5e1fa62762c9aff0aafa66cf196faf9b157f98ec98f59c152fc7bd3",
+      "format": "tar.gz",
+      "path": "package/vendor/x86_64-unknown-linux-musl/zsh/ubuntu-24.04/zsh",
+      "providers": [
+        {
+          "url": "https://github.com/openai/codex/releases/download/rust-v0.104.0/codex-shell-tool-mcp-npm-0.104.0.tgz"
+        },
+        {
+          "type": "github-release",
+          "repo": "openai/codex",
+          "tag": "rust-v0.104.0",
+          "name": "codex-shell-tool-mcp-npm-0.104.0.tgz"
+        }
+      ]
+    },
+    "linux-aarch64": {
+      "size": 53771483,
+      "hash": "blake3",
+      "digest": "ff664f63f5e1fa62762c9aff0aafa66cf196faf9b157f98ec98f59c152fc7bd3",
+      "format": "tar.gz",
+      "path": "package/vendor/aarch64-unknown-linux-musl/zsh/ubuntu-24.04/zsh",
+      "providers": [
+        {
+          "url": "https://github.com/openai/codex/releases/download/rust-v0.104.0/codex-shell-tool-mcp-npm-0.104.0.tgz"
+        },
+        {
+          "type": "github-release",
+          "repo": "openai/codex",
+          "tag": "rust-v0.104.0",
+          "name": "codex-shell-tool-mcp-npm-0.104.0.tgz"
+        }
+      ]
+    },
+  }
+}