Add remote skill sync

[YOMXXX]

Summary

• add a sync-from-origin endpoint for imported skills
• replace skill content/files from the remote source and update origin metadata
• add a skill detail sync action with dirty-state protection

Tests

• cd server && go test ./internal/handler -run 'TestSkillOriginSourceURL|TestImportedSkillFilesFiltersUnsafePaths|TestSyncSkillFromOriginRejectsManualSkill' -count=1
• cd server && go test ./cmd/server -count=1
• pnpm --filter @multica/core typecheck
• pnpm --filter @multica/views typecheck
• jq empty packages/views/locales/en/skills.json packages/views/locales/zh-Hans/skills.json
• git diff --check

[vercel]

@YOMXXX is attempting to deploy a commit to the IndexLabs Team on Vercel.

A member of the Team first needs to authorize it.

[YOMXXX]

Rebased onto current upstream/main and force-pushed 9c7fe79d7; the PR is merge-clean again.

Conflict resolution kept the current skill label routes and added this PR sync route: POST /api/skills/{id}/sync in the same skill detail route group.

I also fixed the frontend CI locale parity failure by adding the missing Japanese strings for the new sync action.

Local verification:

• cd server && go test ./internal/handler -run 'TestSyncSkillFromOriginRejectsManualSkill|TestImportSkill' -count=1
• pnpm --filter @multica/core typecheck
• pnpm --filter @multica/views typecheck
• pnpm --filter @multica/views exec vitest run locales/parity.test.ts
• jq empty packages/views/locales/en/skills.json packages/views/locales/ko/skills.json packages/views/locales/zh-Hans/skills.json packages/views/locales/ja/skills.json
• git diff --check

GitHub CI is green across backend, frontend, mobile, changes, and installers. Ready for review.