Plugin cache accumulates old version directories; no pruning after update

Description

After a plugin updates (e.g. nix-sdlc from 0.29.0 → 0.29.1), the harness correctly writes the new version to cache but leaves the old version directory in place:

~/.claude/plugins/cache/nix-sdlc-marketplace/nix-sdlc/
  0.27.0/   ← stale, never pruned
  0.28.1/   ← stale, never pruned
  0.29.0/   ← stale, never pruned
  0.29.1/   ← current

Steps to Reproduce

1. Install a plugin (e.g. nix-sdlc@0.29.0)
2. Update the plugin to 0.29.1
3. Inspect ~/.claude/plugins/cache/<marketplace>/<plugin>/ — both version directories exist

Impact

Any code or AI agent that resolves the skill path by listing the cache directory (rather than reading installed_plugins.json::installPath) will silently load a stale version. In practice, this caused a failed modernization dry-run where an agent used v0.29.0 skill instructions instead of v0.29.1, missing a critical modernized-app/ isolation rule that was added in the newer version.

Expected Behavior

After a successful update, the harness should remove the previously installed version directory so only one version exists in the cache at any time. Alternatively:

• Document clearly that installed_plugins.json::installPath is the only authoritative resolution path and that listing the cache directory is unsupported
• Add "cache": { "keepOnlyLatest": true } support to plugin.json so plugin authors can opt in to pruning

Workaround

Manually delete old version directories:

rm -rf ~/.claude/plugins/cache/nix-sdlc-marketplace/nix-sdlc/0.29.0

Environment

• Claude Code CLI
• Plugin cache path: ~/.claude/plugins/cache/