docs(plugin-dev): deprecate commands/ in favor of skills/<name>/SKILL.md (#717)

P0 follow-up for EA-471. Updates plugin-dev teaching materials to stop
recommending the commands/ directory layout for new plugins:

- command-development/SKILL.md: add legacy banner at top pointing to
  skills/ format
- create-plugin.md: update scaffolding to create skills/<name>/SKILL.md
  instead of commands/; mark commands/ as acceptable legacy alternative;
  update all examples, tables, and testing instructions
- example-plugin: migrate example-command to skills/example-command/SKILL.md;
  keep commands/example-command.md with a legacy-format note; update README
  to reflect new preferred structure

Both formats remain loaded identically — this is a documentation change only.

Refs: anthropics/apps#26827

Co-authored-by: Henry Shi <henrys@anthropic.com>

plugins/example-plugin/commands/example-command.md

@@ -1,10 +1,12 @@
 ---
-description: An example slash command that demonstrates command frontmatter options
+description: An example slash command that demonstrates command frontmatter options (legacy format)
 argument-hint: <required-arg> [optional-arg]
 allowed-tools: [Read, Glob, Grep, Bash]
 ---
 
-# Example Command
+# Example Command (Legacy `commands/` Format)
+
+> **Note:** This demonstrates the legacy `commands/*.md` layout. For new plugins, prefer the `skills/<name>/SKILL.md` directory format (see `skills/example-command/SKILL.md` in this plugin). Both are loaded identically — the only difference is file layout.
 
 This command demonstrates slash command structure and frontmatter options.