Support external agent config detect and import (#12660)

Migration Behavior

* Config
  *  Migrates settings.json into config.toml
* Only adds fields when config.toml is missing, or when those fields are
missing from the existing file
  *  Supported mappings:
    env -> shell_environment_policy
     sandbox.enabled = true -> sandbox_mode = "workspace-write"

* Skills
  *  Copies home and repo .claude/skills into .agents/skills
  *  Existing skill directories are not overwritten
  *  SKILL.md content is rewritten from Claude-related terms to Codex

* AgentsMd
  *  Repo only
  *  Migrates CLAUDE.md into AGENTS.md
* Detect/import only proceed when AGENTS.md is missing or present but
empty
  *  Content is rewritten from Claude-related terms to Codex

codex-rs/app-server/src/codex_message_processor.rs

@@ -822,6 +822,10 @@ impl CodexMessageProcessor {
             ClientRequest::ConfigRequirementsRead { .. } => {
                 warn!("ConfigRequirementsRead request reached CodexMessageProcessor unexpectedly");
             }
+            ClientRequest::ExternalAgentConfigDetect { .. }
+            | ClientRequest::ExternalAgentConfigImport { .. } => {
+                warn!("ExternalAgentConfig request reached CodexMessageProcessor unexpectedly");
+            }
             ClientRequest::GetAccountRateLimits {
                 request_id,
                 params: _,