[codex] Add Grok via unified ACP flow

[dreamingOf]

Summary

Adds Grok as a first-class ACP agent while keeping it on the existing local ACP registry and binary-cache design.

Details

• Registers Grok in the shared ACP registry as a binary stdio agent (grok agent stdio) with static platform metadata.
• Uses the existing binary-agent install/update/uninstall flow; this PR does not add a Grok-specific remote registry or stable-channel resolver.
• 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, reusing the existing canonical MCP conversion where possible.
• 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 current design.

Review Notes

I specifically checked that the implementation does not retain the earlier Grok-only remote registry path: no GROK_CLI_BASE_URL, stable-channel lookup, dynamic Grok version resolver, or LocalCommand install path remains.

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

[dreamingOf]

Closing per request.