JetBrains: add jet_brains_find_unused_code tool

[MischaPanch]

Summary

Adds a new JetBrains-backend tool, jet_brains_find_unused_code, which reports code symbols
(classes, methods, fields, …) declared in a file that have no references anywhere in the project —
i.e. code that is likely unused/dead.

It is a thin client over a new /findUnusedCode endpoint in the Serena JetBrains plugin: the plugin
walks the IDE's structure-view declaration tree and flags each declaration whose PSI reference search
comes back empty.

Changes

• jetbrains/jetbrains_plugin_client.py: new find_unused_code(relative_path, include_quick_info)
  client method (calls /findUnusedCode; version-gated to plugin 2023.2.17+).
• tools/jetbrains_tools.py: new JetBrainsFindUnusedCodeTool (ToolMarkerSymbolicRead,
  ToolMarkerOptional, ToolMarkerBeta); groups results by file/type, or returns a "none found" message.
• resources/config/internal_modes/jetbrains.yml: register jet_brains_find_unused_code in
  included_optional_tools (so the JetBrains language backend exposes it) and mention it in the mode prompt.
• CHANGELOG.md: entry under Unreleased → JetBrains.

Caveats (documented in the tool description)

This is a usage-based heuristic via the IDE's reference search:

• entry points (e.g. main methods, public API consumed outside the project) and reflective/framework
  usages are not accounted for and may produce false positives;
• transitively-dead code (a private member used only by other dead code) is not reported.

So results should be reviewed before deletion.

Requirements

Requires the Serena JetBrains plugin 2023.2.17+ (which adds the /findUnusedCode endpoint). The
client raises a clear "update the plugin" error against older versions.

Testing

Verified end-to-end against the plugin's headless Docker harness (the serena MCP server built from this
checkout, driving the plugin in IntelliJ): on a sample project the tool reports the one unused private
method and omits the used members, via the full round-trip
MCP client → serena → plugin /findUnusedCode → IntelliJ PSI. poe format and poe type-check pass.

🤖 Generated with Claude Code