[Fix] Add transformToSkillReferences for skills-only delivery mode

[alfred-openspec]

Context

Reported in #879 by @m9dfukc. When delivery: 'skills' is configured, generated SKILL.md files contain hardcoded /opsx:* command references that point to commands that don't exist in skills-only mode.

Root Cause

• Workflow templates in src/core/templates/workflows/*.ts hardcode /opsx:* references (46 total across all templates)
• generateSkillContent() supports a transformInstructions callback, but only transformToHyphenCommands (for OpenCode) exists
• init.ts and update.ts don't pass any transformer when delivery === 'skills'

Proposed Fix

1. Add transformToSkillReferences() to src/utils/command-references.ts using the explicit mapping from WORKFLOW_TO_SKILL_DIR in init.ts:

   • /opsx:apply → /openspec-apply-change
   • /opsx:archive → /openspec-archive-change
   • /opsx:verify → /openspec-verify-change
   • /opsx:continue → /openspec-continue-change
   • /opsx:propose → /openspec-propose
   • /opsx:explore → /openspec-explore
   • /opsx:ff → /openspec-ff-change
   • /opsx:new → /openspec-new-change
   • /opsx:sync → /openspec-sync-specs
   • /opsx:bulk-archive → /openspec-bulk-archive-change
   • /opsx:onboard → /openspec-onboard

2. Wire the transformer in src/core/init.ts and src/core/update.ts:

const transformer = tool.value === 'opencode'
  ? transformToHyphenCommands
  : delivery === 'skills'
    ? transformToSkillReferences
    : undefined;

3. Add tests for the new transformer

Files to Modify

• src/utils/command-references.ts — add transformToSkillReferences()
• src/core/init.ts — wire transformer (~line 541)
• src/core/update.ts — wire transformer (~lines 198, 669)
• Tests

Closes #879