fix(lsp): improve error messages when LSP server is not installed (#305)

Previously, when an LSP server was configured but not installed, the error
message said "No LSP server configured" which was misleading. Now the
error message distinguishes between:

1. Server not configured at all
2. Server configured but not installed (with installation hints)

The new error messages include:
- Clear indication of whether server is configured vs installed
- Installation commands for each built-in server
- Supported file extensions
- Configuration examples for custom servers

Fixes #304

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/tools/lsp/client.ts

@@ -1,9 +1,8 @@
 import { spawn, type Subprocess } from "bun"
 import { readFileSync } from "fs"
 import { extname, resolve } from "path"
-import type { ResolvedServer } from "./config"
 import { getLanguageId } from "./config"
-import type { Diagnostic } from "./types"
+import type { Diagnostic, ResolvedServer } from "./types"
 
 interface ManagedClient {
   client: LSPClient