feat: list available skills when skill:get is run without a name

[mglaman]

What changed

• skill:get now takes name as an optional argument. With no name it lists every skill bundled in skill-data/, showing the name and description from each SKILL.md frontmatter.
• New --format option on skill:get (text|json|md|llm, default text) for the list output. Text is a two-column table plus a Run: drupalorg skill:get <name> hint; llm output is wrapped in <skills> tags.
• Added ListSkillsAction and SkillListResult/SkillItem following the Action/Result/Command layering. The Action parses frontmatter by hand (plain key: value and folded key: > scalars) so the phar needs no YAML dependency. Skill paths are resolved with realpath() and fall back to the raw path inside a phar.
• Registered SkillListResult in AbstractFormatter::format() with a new abstract formatSkillList(), implemented in MarkdownFormatter and LlmFormatter.
• The "Skill not found" hint now reuses the Action instead of a private directory scan in the command.
• Documented the no-argument form in README.md, skill-data/drupalorg-cli/SKILL.md, and the skills/drupalorg-cli/SKILL.md discovery stub.

skill:get <name> and --full behave exactly as before.

Why

Closes #344. drupalorg skill:get with no argument failed with Not enough arguments (missing: "name"), leaving users and agents with no way to discover which skills the installed CLI ships.

Testing

vendor/bin/phpcs src tests
vendor/bin/phpstan analyse src   # [OK] No errors
vendor/bin/phpunit               # OK (181 tests, 601 assertions)

New tests: tests/src/Action/Skill/ListSkillsActionTest.php (reads the real skill-data/), CommandTester cases in tests/src/Command/Skill/GetTest.php for the no-argument, json, llm, named, and unknown-name paths, and formatter cases for SkillListResult.

$ php drupalorg skill:get
+--------------------------------+---------------------------------------------------------------------------------+
| Skill                          | Description                                                                     |
+--------------------------------+---------------------------------------------------------------------------------+
| drupalorg-cli                  | CLI for Drupal.org issue lifecycle management. Use when fetching issue details, |
|                                | generating patches/interdiffs, listing project or maintainer issues, looking up |
...
| drupalorg-work-on-issue        | Agentic workflow for contributing to a Drupal.org issue via GitLab MR.          |
+--------------------------------+---------------------------------------------------------------------------------+

Run: drupalorg skill:get <name>

$ php drupalorg skill:get --format=llm
<skills>
  <usage>drupalorg skill:get &lt;name&gt;</usage>
  <items>
    <skill>
      <name>drupalorg-cli</name>
      <description>CLI for Drupal.org issue lifecycle management. ...</description>
    </skill>
...
</skills>

$ php drupalorg skill:get drupalorg-cli | head -3
---
name: drupalorg-cli
description: >

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