[apiology]

Claude: This PR was written by Claude Code on behalf of @apiology.

Alternative to #260; same behavior, different design.

Problem

A workspace with plugins moves every other workspace to its gem cache:

Solargraph::Workspace.new('/work/rails_app')    # .solargraph.yml: plugins: [solargraph-rails]
plain = Solargraph::Workspace.new('/work/gem')  # no plugins
api_map = Solargraph::ApiMap.new
api_map.catalog Solargraph::Bench.new(workspace: plain)
# expected .../gems, got .../gems-solargraph-rails-1.2.0
Solargraph::CacheDir.gem_dir

In one language server, the plain workspace finds none of its cached gems and rebuilds them into the plugin directory, which castwide/solargraph#1407 introduced.

Solution

CacheDir becomes a class, and each Workspace owns an instance (Workspace#cache_dir) that Config#require_plugins fills with the plugin gems it loads, so the key lives in that object and Collection::Gem, External, ApiMap, the cache/uncache commands and the cache subprocess's --plugins= take it from the workspace they serve.

Convention.register no longer touches the key, because it runs once per process; Convention.add_plugin_conventions(cache_dir, plugin_gems) instead adds the registered conventions defined by that workspace's plugin gems.

A plugin loaded by plain require into a workspace without config is no longer part of any key; it reads and writes the plain gems directory.