Consider adding Java to `lsp.json`

The practice of moving `copilot-sdk-java` to `copilot-sdk` revealed that `lsp.json` does not currently have Java support.

Preliminary research revealed that adding `jdtls` may be a solution, but more research must be done before committing to this approach.

This work item tracks this matter.

Here is some state.

## Assessment: Add Java LSP to lsp.json

### Current State

lsp.json configures language servers for the **Copilot coding agent** (the cloud-based autonomous agent that works in GitHub Actions). It currently provides code intelligence for two languages:

- **C#** — via `dotnet tool run roslyn-language-server` (installed as a .NET tool, scoped to dotnet)
- **Go** — via `gopls serve` (available after `setup-go`, scoped to go)

There is **no Java entry**. This has not been done.

### What does it give the coding agent?

When a language server is configured, the Copilot coding agent gets IDE-quality code intelligence (go-to-definition, find-references, diagnostics, rename) while working autonomously. Without it, the agent relies on text search and heuristics for Java code, which is less reliable for a strongly-typed language with deep class hierarchies.

### Feasibility Concern: jdtls installation

Unlike `gopls` (single binary, already on PATH after `setup-go`) or Roslyn (installable via `dotnet tool`), **Eclipse JDT Language Server (jdtls)** requires:

1. Downloading a release archive (~70MB) from the Eclipse downloads mirror
2. Extracting it to a known location
3. Invoking it with a specific launcher jar path and a configuration directory
4. Providing a `-data` workspace path

This makes it **harder to configure** than the other two, but not impossible.

### Plan

**Option A: Install jdtls in copilot-setup-steps.yml and add to lsp.json** (recommended)

1. **Add a step to copilot-setup-steps.yml** that downloads and extracts jdtls to a known path (e.g., `$HOME/jdtls`):
```yaml
- name: Install Eclipse JDT Language Server
run: |
JDTLS_VERSION="1.44.0"
JDTLS_TIMESTAMP="202501301718"
curl -fsSL "https://www.eclipse.org/downloads/download.php?file=/jdtls/milestones/${JDTLS_VERSION}/jdt-language-server-${JDTLS_VERSION}-${JDTLS_TIMESTAMP}.tar.gz&r=1" \
-o /tmp/jdtls.tar.gz
mkdir -p "$HOME/jdtls"
tar -xzf /tmp/jdtls.tar.gz -C "$HOME/jdtls"
rm /tmp/jdtls.tar.gz
```

2. **Create a wrapper script** at `.github/scripts/jdtls-wrapper.sh` that launches jdtls in stdio mode:
```bash
#!/bin/bash
exec java \
-Declipse.application=org.eclipse.jdt.ls.core.id1 \
-Dosgi.bundles.defaultStartLevel=4 \
-Declipse.product=org.eclipse.jdt.ls.core.product \
-Dlog.level=ALL \
-noverify \
--add-modules=ALL-SYSTEM \
--add-opens java.base/java.util=ALL-UNNAMED \
--add-opens java.base/java.lang=ALL-UNNAMED \
-jar "$HOME/jdtls/plugins/org.eclipse.equinox.launcher_"*.jar \
-configuration "$HOME/jdtls/config_linux" \
-data "$HOME/jdtls-workspace"
```

3. **Update lsp.json** to add a Java entry:
```json
"java": {
"command": "bash",
"args": [".github/scripts/jdtls-wrapper.sh"],
"fileExtensions": {
".java": "java"
},
"rootUri": "java"
}
```

**Option B: Skip this (keep it optional/deferred)**

The coding agent already works on Java code in this repo without an LSP — it uses `mvn verify` for compilation feedback and the `java-coding-skill` SKILL.md for guidance. The agent's productivity without an LSP is "good enough" for the current volume of Java agent work. This could be deferred until the feature is better documented or until a simpler jdtls installation method exists (e.g., a GitHub Action or a single-binary distribution).

### Recommendation

**Go with Option B (defer) for now**, for these reasons:

1. The lsp.json feature itself is not publicly documented — the schema, behavior, and supported options are not well-specified.
2. jdtls installation is fragile (version pinning, platform-specific config directory: `config_linux` vs `config_mac` vs `config_win`, launcher jar glob matching).
3. The agent runner is Ubuntu-only for Copilot cloud agent, so `config_linux` works, but this adds a maintenance burden for version updates.
4. The plan item was marked **"(optional)"** in the migration plan for exactly these reasons.
5. The coding agent already successfully builds and tests Java via `mvn verify` — the LSP would provide incremental improvement, not a critical fix.

### If you want to proceed anyway

The implementation is straightforward — the three changes above (setup step, wrapper script, lsp.json entry). The main risk is jdtls version rot and the fact that jdtls takes 10-30 seconds to initialize a Maven project, which may slow down the agent's startup. I'd suggest opening a separate low-priority issue to track this and revisit when GitHub documents the lsp.json schema officially.

---

Bắt đầu bằng cách đọc lsp.json và copilot-setup-steps.yml, sau đó so sánh các mục language-server hiện có cho C# và Go. Điều tra việc cài đặt jdtls, wrapper, các yêu cầu của launcher và hành vi khởi động của Maven; được xem là hoàn tất khi có cấu hình Java LSP đã được xác thực hoặc có quyết định trì hoãn được ghi lại.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.