Add manual plugin validation fallback when CLI validator is unavailable (#63)

When running inside Cowork, `claude plugin validate` may not be
available. This adds a manual verification checklist covering plugin.json
structure, naming conventions, component directories, and expected file
formats.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

cowork-plugin-management/skills/create-cowork-plugin/SKILL.md

@@ -234,7 +234,14 @@ If the user says "whatever you think is best," provide specific recommendations
 
 1. Summarize what was created — list each component and its purpose
 2. Ask if the user wants any adjustments
-3. Run `claude plugin validate <path-to-plugin-json>`; fix any errors and warnings
+3. Run `claude plugin validate <path-to-plugin-json>` to check the plugin structure. If this command is unavailable (e.g., when running inside Cowork), verify the structure manually:
+   - `.claude-plugin/plugin.json` exists and contains valid JSON with at least a `name` field
+   - The `name` field is kebab-case (lowercase letters, numbers, and hyphens only)
+   - Any component directories referenced by the plugin (`commands/`, `skills/`, `agents/`, `hooks/`) actually exist and contain files in the expected formats — `.md` for commands/skills/agents, `.json` for hooks
+   - Each skill subdirectory contains a `SKILL.md`
+   - Report what passed and what didn't, the same way the CLI validator would
+
+   Fix any errors before proceeding.
 4. Package as a `.plugin` file:
 
 ```bash