feat: add a /help command

[bhushan4work]

What does this change?

Adds /help with one-line descriptions for available commands, conditionally including linking commands when linking is enabled. Includes the repository link for bug reports and ideas.

Closes #1

How I tested this

• npm run typecheck passed
• npm test passed (171 tests, including 3 new help tests)

AI assistance

none

Checklist

• I was assigned the linked issue
• npm run typecheck and npm test pass
• New behavior has a test in test/
• No tokens or secrets in the diff

[anurag629]

Thanks @bhushan4work, this covers everything in the issue. I merged it onto the current main locally: typecheck passes and so do all 178 tests. The descriptions match the registered ones word for word, which I like.

One small thing I'll change myself after merging: the other bot replies don't use the long dash, so I'll swap it for a colon. I'll register the command on the server right after.