Accept a custom conversation store in the server config #114

The README says conversations have "pluggable stores on server", and @chatcops/core has the pieces for it: the ConversationStore interface (packages/core/src/conversation/store.ts) and a store option on ConversationManager. But the server never passes one. createChatHandler calls new ConversationManager() with no arguments (packages/server/src/handler.ts, line 19), and ChatCopsServerConfig has no field for it.

So every deployment keeps its history in memory. It's gone after a restart and isn't shared between instances, which on Cloudflare Workers means between isolates.

What to do

• Add conversationStore?: ConversationStore to ChatCopsServerConfig (packages/server/src/config.ts) and pass it to ConversationManager. maxMessages can come along if you like.
• Without the option, behavior stays exactly as it is.
• Document it in website/src/content/docs/server/setup.mdx and packages/server/README.md, with a short example of a store.

Acceptance criteria

• The config accepts a store and has a JSDoc comment for it; the default is unchanged
• A test proves the handler reads and writes through a supplied store. Use a store that JSON-clones on get and save, like a real database would, so nothing depends on shared object references
• The docs and the server README show the option
• pnpm -r build, pnpm test and pnpm -r typecheck pass
• Changeset for @chatcops/server (minor)

Test note: packages/server/tests/handler.test.ts replaces ConversationManager with a fake, so this test needs its own file that keeps the real @chatcops/core and mocks only createProvider.

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off dev, and open your PR against dev with Closes #<this issue number>. Setup notes (pnpm, build before test) are in CONTRIBUTING.md.