feat(agent-plugins): install project plugins for Claude Code

[Fluzko]

What does this PR do?

Part of #277.
Depends on #332 (until it merges, only the last 3 commits are new here).

Plugins that apply to one project (a workspace use, a registry plugin active through its depends-on, a workspace member's skills, a consented dependency) reached Claude Code as loose skill folders with no plugin identity. Now each one is compiled into the project's .symposium/plugins/ and reaches Claude Code as a plugin in .claude/skills/<plugin>/, invoked as /<plugin>:<skill> in that project only. No Claude settings are written and git status stays clean. Other agents keep per-project skill copies.

Verified against Claude Code 2.1.284, including two repositories with the same plugin name open at once and an upgrade from the old skill folders.

Take into account:

• Claude Code loads project plugins only from the folder it starts in, so a session started in a subdirectory gets none of them (agent-plugins: Claude Code does not load project plugins in workspace subdirectories #353).
• They load only in trusted folders; the session that accepts the trust prompt sees them after /reload-plugins.
• A global plugin with the same name hides the project plugin.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code