Plugin: add a /codex_new flow for new threads

[huntharo]

Problem

We need a /codex_new flow that feels like the sibling of /codex_resume, but starts a fresh Codex thread instead of rebinding an existing one.

If worktrees land, this becomes a major entrypoint rather than just a convenience wrapper.

Current Findings

• The plugin already has project/thread picker flows for /codex_resume.
• Codex App Server supports starting new threads with explicit cwd and config overrides.
• I did not find an App Server protocol method for "add this directory as a new project folder" or a first-class project-management RPC in the same sense as Desktop's project list UX.
• I also did not find protocol-level worktree lifecycle support, so richer /codex_new behavior would likely need client/plugin-owned worktree creation.

Proposed Scope

• Add /codex_new to present a project picker.
• Start a new thread in the selected project/worktree and optionally bind immediately to the current conversation.
• If/when worktrees land, let /codex_new optionally create a worktree before thread start.
• Evaluate whether we can expose "add a new directory" in a sane way, or whether that belongs in OpenClaw/core configuration rather than the Codex App Server protocol.

Source Notes

• Existing resume/project picker command flow: src/controller.ts:982
• Thread start support in App Server: codex-rs/app-server-protocol/schema/typescript/ClientRequest.ts:56
• Thread start params (cwd, config overrides): codex-rs/app-server-protocol/schema/typescript/v2/ThreadStartParams.ts:10
• Plugin thread picker/project grouping: src/thread-picker.ts:12