feat(plugins): support monorepo subdirectory sources in the marketplace

Problem

The plugin marketplace cannot install a plugin that lives in a monorepo subdirectory.

resolveInstallSource (packages/agent-core-v2/src/app/plugin/source.ts:63) joins every path segment after /tree/ into the git ref:

if (head === 'tree' && rest.length >= 2) {
  const refValue = decodeRefSegments(rest.slice(1)); // all segments -> ref
  ...
}

So https://github.com/owner/repo/tree/main/path/to/plugin resolves to ref main/path/to/plugin, and codeload fetches the wrong archive instead of checking out main and extracting path/to/plugin. Confirmed by the Codex review on #2944.

Why it matters

Any plugin hosted in a monorepo subdirectory (common when one plugin is shared across several agent hosts) cannot be listed without forking it into a standalone repo, which fragments a single source of truth. This affects monorepo-hosted plugins generally.

Proposal: encode the subdirectory in the source string (following pip / go-getter)

The established convention for "repo + subdirectory + ref" encodes the subdir in the source string with an explicit delimiter, avoiding the ref-vs-subdir ambiguity (a branch name can contain slashes, which is why the current code joins the whole tail as the ref):

• pip / PEP 508: git+https://github.com/owner/repo@ref#subdirectory=path (pip docs)
• Terraform / go-getter: github.com/owner/repo//modules/vpc?ref=v1.2.0 (Terraform module sources)

For Kimi's GitHub web-URL source form, the pip-style #subdirectory= fragment is the cleanest fit because it rides on the existing new URL() parse with no change to ref handling:

https://github.com/mem0ai/mem0/tree/main#subdirectory=integrations/mem0-plugin

Implementation:

• source.ts: parseGithubUrl already builds a URL, so read url.hash (#subdirectory=<path>) and carry an optional subdir on the github ResolvedSource. The existing /tree/<ref> parsing is unchanged (url.pathname still yields ref main).
• manager.ts (after extractZip): sourceRoot = path.join(sourceRoot, subdir) before parseManifest, and persist subdir on the plugin record so updates reuse it.

The marketplace entry stays a single source field:

{ "id": "mem0", "tier": "curated", "source": "https://github.com/mem0ai/mem0/tree/main#subdirectory=integrations/mem0-plugin" }

This keeps source a single string (consistent with the existing local-path / zip-url / github-url handling), matches a widely used convention, and is unambiguous. (A separate subdir entry field would also work, but the URL-encoded form is more consistent with the single-string source model.)

Offer

Happy to contribute the PR (source.ts + manager.ts + tests + docs + changeset). Relates to #2944 (adding a monorepo-hosted plugin to the marketplace). Which syntax do you prefer before I open it?

Start with parseGithubUrl and resolveInstallSource in packages/agent-core-v2/src/app/plugin/source.ts, then inspect manager.ts around extractZip and parseManifest. Check the existing plugin-source tests and record shape before deciding how the optional subdirectory is carried through. Done means a GitHub tree URL can target a monorepo subdirectory while preserving the ref and update behavior.