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/skills/example-command/SKILL.md

@@ -0,0 +1,39 @@
+---
+name: example-command
+description: An example user-invoked skill that demonstrates frontmatter options and the skills/<name>/SKILL.md layout
+argument-hint: <required-arg> [optional-arg]
+allowed-tools: [Read, Glob, Grep, Bash]
+---
+
+# Example Command (Skill Format)
+
+This demonstrates the `skills/<name>/SKILL.md` layout for user-invoked slash commands. It is functionally identical to the legacy `commands/example-command.md` format — both are loaded the same way; only the file layout differs.
+
+## Arguments
+
+The user invoked this with: $ARGUMENTS
+
+## Instructions
+
+When this skill is invoked:
+
+1. Parse the arguments provided by the user
+2. Perform the requested action using allowed tools
+3. Report results back to the user
+
+## Frontmatter Options Reference
+
+Skills in this layout support these frontmatter fields:
+
+- **name**: Skill identifier (matches directory name)
+- **description**: Short description shown in /help
+- **argument-hint**: Hints for command arguments shown to user
+- **allowed-tools**: Pre-approved tools for this skill (reduces permission prompts)
+- **model**: Override the model (e.g., "haiku", "sonnet", "opus")
+
+## Example Usage
+
+```
+/example-command my-argument
+/example-command arg1 arg2
+```