feat(plugins): scaffold git, gitea, and core plugins

Adds three new plugins to the holocron marketplace:
- git: 4 stub skills (git-commit, git-branch, git-pr, git-flow)
- gitea: 5 stub skills (gitea-issues, gitea-prs, gitea-milestones, gitea-releases, gitea-wiki)
- core: 5 stub skills (triage, diagnose, zoom-out, caveman, improve-codebase-architecture)

All plugins follow version-parity convention (ADR-0016). Skills are
placeholder stubs — content will be authored in a future workstream.
Registered in both marketplace.json paths.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/core/skills/improve-codebase-architecture/SKILL.md

@@ -0,0 +1,11 @@
+---
+name: improve-codebase-architecture
+description: >
+  Use when you want to identify and improve structural weaknesses in the
+  codebase architecture — coupling, cohesion, layering, or boundary violations.
+  Stub — full content not yet written.
+metadata:
+  category: cross-cutting
+---
+
+> **Stub.** This skill is a placeholder. Content will be authored in a future workstream.