feat(labels): Make labels configurable with enable/disable and custom colors

[myakove]

Summary

Add configuration options to:

1. Enable/disable specific labels per repository
2. Customize label colors via CSS3 color names
3. Maintain backward compatibility (no config = current behavior)

Proposed Configuration

labels:
  # If not set, ALL labels are enabled (backward compatible)
  enabled-labels:
    - verified
    - hold
    - wip
    - needs-rebase
    - has-conflicts
    - can-be-merged
    - size              # All size/* labels
    - branch            # branch-* labels
    - cherry-pick       # cherry-pick-* labels
    - automerge

  # Optional: Override default colors (CSS3 color names)
  colors:
    hold: red
    verified: green
    approved-: green
    lgtm-: yellowgreen
    # ... etc

Label Categories

Category	Labels	Configurable
Status	verified, hold, wip, needs-rebase, has-conflicts, can-be-merged, automerge	Enable/Disable + Colors
Size	size/XS, size/S, etc.	Enable/Disable + Colors
Branch	branch-{name}	Enable/Disable + Colors
Reviewed-by	approved-, lgtm-, changes-requested-, commented-	Colors ONLY (cannot disable)
Cherry-pick	cherry-pick-*, CherryPicked	Enable/Disable + Colors

Important: reviewed-by Labels Are Always Enabled

The reviewed-by labels cannot be disabled because they are the only source of truth for the approval system. Disabling them would break:

• minimum-lgtm requirement (counts lgtm-* labels)
• Approval validation (counts approved-* labels)
• Changes-requested blocking (checks changes-requested-* labels)
• Auto-merge workflow

Files to Modify

1. webhook_server/config/schema.yaml - Add labels schema
2. webhook_server/utils/constants.py - Remove hardcoded colors
3. webhook_server/libs/github_api.py - Load labels config
4. webhook_server/libs/handlers/labels_handler.py - Check enabled + use configured colors
5. webhook_server/libs/handlers/pull_request_handler.py - Conditional label operations
6. webhook_server/libs/handlers/issue_comment_handler.py - Conditional /wip, /hold, /verified
7. Tests and documentation

Acceptance Criteria

• Labels can be enabled/disabled via config (except reviewed-by)
• Label colors can be customized via CSS3 color names
• No config = all labels enabled with default colors (backward compatible)
• Schema validation works correctly
• All tests pass
• Documentation updated