[codex] Add Grok ACP agent support

[dreamingOf]

Summary

Adds Grok as a first-class ACP agent using the same binary cache, install, upgrade, uninstall, settings, MCP, skills, and conversation history paths used by the existing agents.

Details

• Registers Grok in the shared ACP registry as a binary stdio agent (grok agent stdio).
• Uses the official Grok stable channel to resolve remote versions and download platform binaries into the app binary cache.
• Extends binary install support for raw executable downloads in addition to archives.
• Adds Grok MCP support through ~/.grok/config.toml with the CLI-compatible [mcp_servers.<name>] shape.
• Adds a Grok local transcript parser for ~/.grok/sessions, so reopened sessions can show user/assistant turns instead of an empty conversation.
• Adds frontend type/icon/settings coverage and hides the config-management block for Grok to match the agent's design.

Validation

• cargo fmt --check && cargo check
• cargo test grok --lib
• corepack pnpm eslint src/components/settings/acp-agent-settings.tsx src/components/settings/mcp-settings.tsx src/components/agent-icon.tsx src/lib/types.ts
• corepack pnpm build
• git diff --check