refactor: shared onboarding helper + consistent init/update output

[1code-async]

Summary

• Root cause: init and update had divergent onboarding output — update hardcoded /opsx:new regardless of profile, while init correctly chose /opsx:propose for core profile. Both commands also copy-pasted removeSkillDirs/removeCommandFiles and the WORKFLOW_TO_SKILL_DIR constant independently.
• Fix: Extract shared helpers (onboarding.ts, artifact-removal.ts) so both commands produce consistent, profile-aware output from a single source of truth.

What changed

File	Change
src/core/shared/onboarding.ts	New. formatGettingStarted, printOnboardingFooter, formatIdeRestart, formatLinks
src/core/shared/artifact-removal.ts	New. removeAllSkillDirs, removeUnselectedSkillDirs, removeAllCommandFiles, removeUnselectedCommandFiles
src/core/shared/index.ts	Re-exports new modules
src/core/init.ts	Uses shared helpers; removes duplicate WORKFLOW_TO_SKILL_DIR, 2 private removal methods, inline onboarding footer
src/core/update.ts	Uses shared helpers; removes 4 private removal methods, inline onboarding block; fixes legacy-upgrade to respect active profile
test/core/update.test.ts	Updates legacy-upgrade assertion from /opsx:new → /opsx:propose (matches corrected behavior)
test/core/shared-onboarding.test.ts	New. 10 tests covering formatGettingStarted, formatLinks, formatIdeRestart, printOnboardingFooter
test/core/output-consistency.test.ts	New. 7 tests verifying init/update/migration produce consistent onboarding output

Net effect

• init.ts: 779 → 707 lines (−72)
• update.ts: 702 → 583 lines (−119)
• Shared code: +2 new modules (≈200 lines total)
• Tests: +17 new tests, all 122 existing tests pass

Follow-ups (not in this PR)

1. Centralize summary formatting — init shows "Created/Refreshed" while update shows "✓ Updated (vX.Y.Z)". A shared formatUpdateSummary() would make these consistent too.
2. Extract tool-selection logic — init and update both independently handle tool detection, pre-selection, and validation. A shared ToolSelector class would prevent future drift.
3. Document intentional legacy-cleanup behavior difference — init aborts if user refuses cleanup; update warns but continues. This is by design but undocumented.

Test plan

• shared-onboarding.test.ts: All profile combinations produce correct commands
• output-consistency.test.ts: init and update produce identical IDE-restart wording, correct links, profile-aware getting-started
• init.test.ts: All 26 existing tests pass (no regressions)
• update.test.ts: All 42 tests pass (1 assertion updated to match fixed behavior)
• migration.test.ts: All 7 tests pass

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