Retire legacy `spectral` commit scope

[andreyyudin]

Description

Retire the legacy spectral commit scope that was temporarily retained during the Spectral → Vacuum migration work in #171.

This came out of PR discussion in #171: the old scope was intentionally left in place so existing branch history and in-flight commits would not fail commit linting mid-migration, but it should now be removed so active configuration and contributor guidance match the current Vacuum-based terminology.

Benefits

• Aligns commit conventions with the current ruleset / linting-rules terminology.
• Removes a stale scope that suggests Spectral is still an active tool in this repository.
• Reduces confusion for contributors writing new commits.
• Completes a follow-up cleanup explicitly identified during PR review.

Considerations

• Merged historic commits should not be rewritten.
• Any in-flight branches, queued work, local templates, or automation still using the spectral scope will need remediation.
• Contributor guidance should make clear which replacement scope(s) to use for future work.

Acceptance Criteria

• spectral is removed from the active allowed commit scopes.
• Contributor docs/examples no longer present spectral as a current valid scope.
• ruleset and/or linting-rules are documented as the canonical replacement scopes.
• The repository is checked for remaining references that imply spectral is still an active commit scope.
• The issue notes that existing in-flight work using spectral may need rebasing or commit amendment to comply with current commit linting expectations.

Proposed Steps

1. Remove spectral from commitlint configuration.
2. Search the repository for contributor guidance or examples that still mention spectral as a valid commit scope.
3. Update those references to use ruleset / linting-rules consistently.
4. Add a short note in the relevant contributor guidance about remediating in-flight branches or pending work that still uses the old scope.
5. Confirm there are no remaining active references that imply spectral is still the preferred scope.