Add lsp-setup skill for configuring LSP servers in Copilot CLI

Adds a new skill that helps users install and configure Language Server
Protocol servers for GitHub Copilot CLI. Includes:

- Interactive workflow: detect OS, install server, write config, verify
- Bundled reference with 14 languages (Java/jdtls, TypeScript, Python,
  Go, Rust, C/C++, C#, Ruby, PHP, Kotlin, Swift, Lua, YAML, Bash)
- Per-OS install commands and ready-to-use lsp-config.json snippets
- Support for both user-level and repo-level configuration

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

skills/lsp-setup/SKILL.md

@@ -0,0 +1,69 @@
+---
+name: lsp-setup
+description: 'Install and configure LSP servers for any programming language to work with GitHub Copilot CLI. Detects the OS, installs the right LSP server, and writes the lsp-config.json configuration. Say "setup LSP" to start.'
+---
+
+# LSP Setup for GitHub Copilot CLI
+
+**UTILITY SKILL** — installs and configures Language Server Protocol servers for Copilot CLI.
+USE FOR: "setup LSP", "install language server", "configure LSP for Java", "add TypeScript LSP", "enable code intelligence"
+DO NOT USE FOR: general coding tasks, IDE/editor LSP configuration, non-Copilot-CLI setups
+
+## Workflow
+
+1. **Ask the language** — use `ask_user` to ask which programming language(s) the user wants LSP support for
+2. **Detect the OS** — run `uname -s` (or check for Windows via `$env:OS` / `%OS%`) to determine macOS, Linux, or Windows
+3. **Look up the LSP server** — read `references/lsp-servers.md` for known servers, install commands, and config snippets
+4. **Ask scope** — use `ask_user` to ask whether the config should be user-level (`~/.copilot/lsp-config.json`) or repo-level (`.github/lsp.json`)
+5. **Install the server** — run the appropriate install command for the detected OS
+6. **Write the config** — merge the new server entry into the chosen config file (create it if missing, preserve existing entries)
+7. **Verify** — confirm the LSP binary is on `$PATH` and the config file is valid JSON
+
+## Configuration Format
+
+Copilot CLI reads LSP configuration from two locations (repo-level takes precedence):
+
+- **User-level**: `~/.copilot/lsp-config.json`
+- **Repo-level**: `.github/lsp.json`
+
+The JSON structure:
+
+```json
+{
+  "lspServers": {
+    "<server-key>": {
+      "command": "<binary>",
+      "args": ["--stdio"],
+      "fileExtensions": {
+        ".<ext>": "<languageId>",
+        ".<ext2>": "<languageId>"
+      }
+    }
+  }
+}
+```
+
+### Key rules
+
+- `command` is the binary name (must be on `$PATH`) or an absolute path.
+- `args` almost always includes `"--stdio"` to use standard I/O transport.
+- `fileExtensions` maps each file extension (with leading dot) to a [Language ID](https://code.visualstudio.com/docs/languages/identifiers#_known-language-identifiers).
+- Multiple servers can coexist in `lspServers`.
+- When merging into an existing file, **never overwrite** other server entries — only add or update the target language key.
+
+## Behavior
+
+- Always use `ask_user` with `choices` when asking the user to pick a language or scope.
+- If the language is not listed in `references/lsp-servers.md`, search the web for "<language> LSP server" and guide the user through manual configuration.
+- If a package manager is not available (e.g. no Homebrew on macOS), suggest alternative install methods from the reference file.
+- After installation, run `which <binary>` (or `where.exe` on Windows) to confirm the binary is accessible.
+- Show the user the final config JSON before writing it.
+- If the config file already exists, read it first and merge — do not clobber.
+
+## Verification
+
+After setup, tell the user:
+
+1. Launch `copilot` in a project with files of the configured language
+2. Run `/lsp` to check the server status
+3. Try code intelligence features like go-to-definition or hover