feat(PE-6804): transaction proposal

[0xdiid]

Description

This adds transaction proposal to the CLI, dependent on the relevant backend PR

Changes made

• Add transaction create subgroup with "custom" and "transfer" options (swaps, bridges, etc. will go here in the future)
• Add transaction cancel call

How has this been tested?

You can test it with the server running locally. Here's how I have it in my .claude.json:

      "mcpServers": {
        "splits-dev": {
          "command": "pnpx",
          "args": [
            "tsx",
            "/Users/willdrach/Git/splits-workspace/splits-cli/src/cli.ts",
            "--mcp"
          ],
          "env": {
            "SPLITS_API_KEY": "sk_<your key>",
            "SPLITS_API_URL": "http://localhost:8080"
          }
        }
      }

Other notes