feat: add plugin-create skill and plugin template

Adds the plugin-create skill (.agents/skills/plugin-create/) that scaffolds
a new plugin from templates/plugin/, substitutes PLUGIN_NAME/AUTHOR_* markers,
registers the plugin in marketplace.json, runs claude plugin validate ., and
hands off to /marketplace-architect. Includes evals and the generic template
with all components (agents, skills, hooks, MCP, bin, docs) with per-directory
READMEs explaining what goes where and which tool reads each directory.

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

templates/plugin/skills/README.md

@@ -0,0 +1,18 @@
+# skills/
+
+Skills for this plugin. Each skill lives in its own subdirectory:
+
+```
+skills/
+  <skill-name>/
+    SKILL.md      # required — frontmatter + skill body
+    META.md       # required — provenance and audit fields
+    references/   # optional — on-demand reference docs
+    scripts/      # optional — executable helper scripts
+    assets/       # optional — templates, data files, lookup tables
+```
+
+**Shared** — both Claude Code and GitHub Copilot CLI read `skills/<name>/SKILL.md`.
+
+To add a skill, run `/write-skill` in a Claude Code session. Do not write SKILL.md by hand
+without following the authoring standard — trigger descriptions and self-checks are required.