Memory leak: LSP client diagnostics and files maps not cleared on shutdown

[sauerdaniel]

Problem

When an LSP client shuts down, the diagnostics Map and files object are not cleared in the shutdown() method of LSPClient. This can cause:

1. Memory retention - Diagnostics data for closed files remains in memory
2. Stale data - If the same LSP server is reconnected, old diagnostics may persist
3. Memory growth - Over multiple LSP reconnections, maps accumulate entries

Code Location

packages/opencode/src/lsp/client.ts - The shutdown() method closes the connection but doesn't clear internal state.

Proposed Solution

Add cleanup calls in the shutdown() method:

diagnostics.clear()
for (const path in files) delete files[path]

Impact

• Minimal change (2 lines)
• Ensures clean state on LSP shutdown
• Prevents memory growth from accumulated diagnostics

Relates to #5363