Support detect and import MCP, Subagents, hooks, commands from external (#19949)

## Why
This PR expands the migration path so Codex can detect and import MCP
server config, hooks, commands, and subagents configs in a Codex-native
shape.

## What changed

- Added a `codex-external-agent-migration` crate that owns conversion
logic for external-agent MCP servers, hooks, commands, and subagents.
- Extended the app-server external-agent config detection/import API
with migration item types for MCP server config, hooks, commands, and
subagents.

## Migration strategy

The migration is intentionally conservative: Codex only imports
external-agent config that can be represented safely in Codex today.
Unsupported or ambiguous config is skipped instead of being partially
translated into behavior that may not match the source system.

- **MCP servers**: import supported stdio and HTTP MCP server
definitions into `mcp_servers`. Disabled servers and servers filtered
out by source `enabledMcpjsonServers` / `disabledMcpjsonServers` are
skipped. Project-scoped MCP entries from `.claude.json` are included
when they match the repo path.
- **Hooks**: import only supported command hooks into
`.codex/hooks.json`. Unsupported hook features such as conditional
groups, async handlers, prompt/http hooks, or unknown fields are
skipped. Referenced hook scripts are copied into `.codex/hooks/`,
preserving any existing target scripts.
- **Commands**: import supported external commands as Codex skills under
`.agents/skills/source-command-*`. Commands that rely on source runtime
expansion such as `$ARGUMENTS`, `$1`, `@file` references, shell
interpolation, or colliding generated names are skipped.
- **Subagents**: import valid subagent Markdown files into
`.codex/agents/*.toml` when they have the minimum Codex agent fields.
Source model names are not migrated, so imported agents keep the user’s
Codex default model; compatible reasoning effort and sandbox mode are
migrated when present.
- **Skills and project guidance**: copy missing skill directories into
`.agents/skills` and migrate `CLAUDE.md` guidance into `AGENTS.md`,
rewriting source-agent terminology to Codex terminology where
appropriate.
- **Detection details**: detected migration items include lightweight
details for UI preview, such as MCP server names, hook event names,
generated command skill names, and subagent names. Import still
recomputes from disk instead of trusting details as the source of truth.

- Adds focused coverage for the new migration behavior and app-server
import flow.

## Verification

- `cargo test -p codex-external-agent-migration`
- `cargo test -p codex-hooks`
- `cargo test -p codex-app-server external_agent_config`
- `just bazel-lock-check`

codex-rs/app-server-protocol/schema/json/v2/ExternalAgentConfigDetectResponse.json

@@ -1,6 +1,17 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
+    "CommandMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
+    },
     "ExternalAgentConfigMigrationItem": {
       "properties": {
         "cwd": {
@@ -40,12 +51,58 @@
         "SKILLS",
         "PLUGINS",
         "MCP_SERVER_CONFIG",
+        "SUBAGENTS",
+        "HOOKS",
+        "COMMANDS",
         "SESSIONS"
       ],
       "type": "string"
     },
+    "HookMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
+    },
+    "McpServerMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
+    },
     "MigrationDetails": {
       "properties": {
+        "commands": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/CommandMigration"
+          },
+          "type": "array"
+        },
+        "hooks": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/HookMigration"
+          },
+          "type": "array"
+        },
+        "mcpServers": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/McpServerMigration"
+          },
+          "type": "array"
+        },
         "plugins": {
           "default": [],
           "items": {
@@ -59,6 +116,13 @@
             "$ref": "#/definitions/SessionMigration"
           },
           "type": "array"
+        },
+        "subagents": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/SubagentMigration"
+          },
+          "type": "array"
         }
       },
       "type": "object"
@@ -101,6 +165,17 @@
         "path"
       ],
       "type": "object"
+    },
+    "SubagentMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
     }
   },
   "properties": {

codex-rs/app-server-protocol/schema/json/v2/ExternalAgentConfigImportParams.json

@@ -1,6 +1,17 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
+    "CommandMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
+    },
     "ExternalAgentConfigMigrationItem": {
       "properties": {
         "cwd": {
@@ -40,12 +51,58 @@
         "SKILLS",
         "PLUGINS",
         "MCP_SERVER_CONFIG",
+        "SUBAGENTS",
+        "HOOKS",
+        "COMMANDS",
         "SESSIONS"
       ],
       "type": "string"
     },
+    "HookMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
+    },
+    "McpServerMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
+    },
     "MigrationDetails": {
       "properties": {
+        "commands": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/CommandMigration"
+          },
+          "type": "array"
+        },
+        "hooks": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/HookMigration"
+          },
+          "type": "array"
+        },
+        "mcpServers": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/McpServerMigration"
+          },
+          "type": "array"
+        },
         "plugins": {
           "default": [],
           "items": {
@@ -59,6 +116,13 @@
             "$ref": "#/definitions/SessionMigration"
           },
           "type": "array"
+        },
+        "subagents": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/SubagentMigration"
+          },
+          "type": "array"
         }
       },
       "type": "object"
@@ -101,6 +165,17 @@
         "path"
       ],
       "type": "object"
+    },
+    "SubagentMigration": {
+      "properties": {
+        "name": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "type": "object"
     }
   },
   "properties": {