docs: add a Troubleshooting section to the README

[theyashgupta]

First-run snags all land in the same few places. Collect them into a short Troubleshooting section in README.md so new users can self-serve.

Suggested entries

• A required tool is missing. Dispatch names it at startup; point to the install steps in Getting started.
• Port already in use. The backend binds port from ~/.dispatch/config.json (default 4700); change it and restart.
• No tickets show up. Check linearApiKey is set in ~/.dispatch/config.json, and that tickets are assigned to you in an unstarted state.

Docs only, no code changes. Keep it tight.

Why it's a good first issue

Small, self-contained, and verifiable by reading. A great way to get familiar with how Dispatch starts up.