Plugin install with source-type git-subdir creates empty destination directory (subdir contents not copied)

Summary

When a plugin uses source: { source: "git-subdir", url: ..., path: ..., ref: ... } in its marketplace manifest, /plugin install creates the destination directory but does not populate it with the contents of the specified subdir. The marketplace itself loads fine (schema validation passes — distinct from already-resolved schema bugs in #33828, #36848, #40595, #33172, etc.).

This is downstream of the marketplace clone — that step works correctly here. The failure is in the plugin install step's subdir-copy phase.

Environment

• Claude Code v2.1.139
• macOS (Darwin 24.6.0, Apple Silicon)
• zsh

Repro

Marketplace manifest at https://github.com/FirstPersonSF/cp-engine (repo root) → .claude-plugin/marketplace.json:

{
  "name": "cp-engine",
  "version": "0.8.0",
  "description": "Slash commands for the Context Protocol Engine.",
  "owner": {
    "name": "First Person + Canonic",
    "url": "https://github.com/FirstPersonSF"
  },
  "plugins": [
    {
      "name": "cp-engine",
      "description": "Capture development sessions back to the Context Protocol working tree.",
      "author": { "name": "First Person + Canonic" },
      "category": "development",
      "source": {
        "source": "git-subdir",
        "url": "https://github.com/FirstPersonSF/cp-engine.git",
        "path": "plugin",
        "ref": "main"
      },
      "homepage": "https://github.com/FirstPersonSF/cp-engine"
    }
  ]
}

The plugin/ subdir at the repo root contains:

plugin/
├── plugin.json
├── README.md
├── commands/
│   ├── cp-context.md
│   └── cp-summarize.md
└── hooks/
    ├── hooks.json
    └── sync-cli-version.sh

Steps:

1. Add the marketplace: /plugin marketplace add FirstPersonSF/cp-engine
2. Install the plugin: /plugin install cp-engine@cp-engine
3. Observe: ~/.claude/plugins/data/cp-engine-cp-engine/ exists but is empty.
4. Reload plugins (/plugin reload); the plugin's slash commands (/cp-context, /cp-summarize) do not appear.

Expected

After install, ~/.claude/plugins/data/cp-engine-cp-engine/ should contain the contents of the repo's plugin/ subdir at ref main — i.e. plugin.json, README.md, commands/, hooks/ — and the slash commands should be loaded.

Workaround

Manually clone the repo and copy the subdir contents:

git clone https://github.com/FirstPersonSF/cp-engine.git /tmp/cp-engine
cp -R /tmp/cp-engine/plugin/* ~/.claude/plugins/data/cp-engine-cp-engine/

Then /plugin reload. Commands appear correctly.

Why this seems to be a real install-flow bug, not a config issue

• The marketplace clone at ~/.claude/plugins/marketplaces/cp-engine/ is populated correctly — the underlying git access works.
• The same repo, same ref, manually cloned + copied, produces a fully functional plugin (verified — commands appear after /plugin reload).
• The destination directory is created during install (so the code path knows to do something); only the file copy is missing.
• Schema validation passes (no errors surfaced during install — distinct from prior git-subdir schema bugs that produced explicit validation errors).

Related but distinct

• #45653 (marketplace git clones empty) — different layer; that's the registry clone failing. Here the registry succeeds; the plugin subdir copy fails.
• #33828 / #36848 / #40595 / #33172 — closed git-subdir schema-validation bugs. Schema validation works for me; this is a runtime install-flow bug.