feat(copilot): support `--integration-options="--skills"` for skills-based scaffolding (#2324)

* Initial plan

* feat(copilot): add --skills flag for skills-based scaffolding

Add --skills integration option to CopilotIntegration that scaffolds
commands as speckit-<name>/SKILL.md under .github/skills/ instead of
the default .agent.md + .prompt.md layout.

- Add options() with --skills flag (default=False)
- Branch setup() between default and skills modes
- Add post_process_skill_content() for Copilot-specific mode: field
- Adjust build_command_invocation() for skills mode (/speckit-<stem>)
- Update dispatch_command() with skills mode detection
- Parse --integration-options during init command
- Add 22 new skills-mode tests
- All 15 existing default-mode tests continue to pass

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* docs(AGENTS.md): document Copilot --skills option

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* Potential fix for pull request finding 'Unused local variable'

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* fix: address PR #2324 review feedback

- Reset _skills_mode at start of setup() to prevent singleton state leak
- Tighten skills auto-detection to require speckit-*/SKILL.md (not any
  non-empty .github/skills/ directory)
- Add copilot_skill_mode to init next-steps so skills mode renders
  /speckit-plan instead of /speckit.plan
- Fix docstring quoting to match actual unquoted output
- Add 4 tests covering singleton reset, auto-detection false positive,
  speckit layout detection, and next-steps skill syntax
- Fix skipped test_invalid_metadata_error_returns_unknown by simulating
  InvalidMetadataError on Python versions that lack it

* fix: inline skills prompt in dispatch_command auto-detection path

build_command_invocation() reads self._skills_mode which stays False
when skills mode is only auto-detected from the project layout. Inline
the /speckit-<stem> prompt construction so dispatch_command() sends the
correct prompt regardless of how skills mode was detected.

Also strengthen test_dispatch_detects_speckit_skills_layout to assert
the -p prompt contains /speckit-plan and the user args.

---------

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Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>