Deduplicate skill file via symlink (#32)

## Summary

- Replaces `skills/write-fixtures/SKILL.md` (full copy) with a symlink
to `.claude/commands/write-fixtures.md`
- Single source of truth, both `--plugin-dir` and `--add-dir` discovery
paths still work

## Test plan

- [ ] `claude --plugin-dir ./node_modules/@copilotkit/llmock` discovers
`/llmock:write-fixtures`
- [ ] `claude --add-dir ./node_modules/@copilotkit/llmock` discovers
`/write-fixtures`
- [ ] Symlink resolves correctly after `npm install`

Author: jpr5

skills/write-fixtures/SKILL.md

@@ -0,0 +1 @@
+../../.claude/commands/write-fixtures.md