Add JSON output for skill-validator check (#601)

* Add JSON output for skill-validator check

Fixes #600

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix empty check discovery results

Address PR feedback by failing when explicit skill or agent paths discover nothing, including the combined check path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address Copilot review comments

- Replace AddPlainError with AddGeneralError (remove duplicate method)
- Key pluginSkills by DirectoryPath instead of display name to avoid
  mismatch when plugin.Name differs from directory name
- Use OS-aware path comparison in IsPathWithin (Ordinal on Unix,
  OrdinalIgnoreCase on Windows)
- Pre-build name lookup dictionaries in CreateJsonOutput to eliminate
  O(n*m) FirstOrDefault scans for external dependency attachment
- Extract CheckJsonSerializerContext scoped to check JSON output so
  UseStringEnumConverter does not affect unrelated JSON payloads

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address latest Copilot PR feedback

- Attach external dependency warnings by stable path identifiers
  (skill SKILL.md path, agent path, plugin directory path)
- Use OS-aware comparer for plugin directory path matching
- Restore console warning formatting via SkillProfiler.FormatProfileWarnings
- Move CheckJsonSerializerContext into SkillValidator.Check namespace
- Precompute reference-attachment targets to avoid repeated full-path
  normalization and container recalculation per finding
- Add regression test covering duplicate skill names with external
  dependency warnings in JSON output

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix flaky duplicate-skill JSON test path

- Run duplicate-name external dependency assertion through plugin mode,
  which is where external dependency checks are executed
- Add plugin fixture helper for JSON output tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address latest Copilot follow-up feedback

- Reintroduce PluginValidationResult as obsolete compatibility type
  while keeping PluginCheckResult as the primary model
- Avoid unnecessary profile-line formatting when verbose output is off
- Replace string discriminators for external dependency kind with enum
- Centralize JSON warning kind literals as constants

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aaronpowell