Refactor link command into unified pipeline orchestrator

[munezaclovis]

Summary

• All link-time operations are now Steps flowing through a single RunPipeline call — PHP install, Caddyfile generation, TLS certs, service detection, and all Laravel-specific steps
• Critical() bool added to Step interface — critical step failures abort the pipeline, non-critical failures continue with a visual ✗
• cmd/link.go reduced from 250 to 170 lines — thin orchestrator that builds context, assembles step list, runs one pipeline
• Hidden automation gates (generate_site_config, generate_caddyfile, generate_tls_cert, detect_services) — configurable via pv.yml but not shown in setup wizard
• Octane re-detection moved into InstallOctaneStep.Run() — step owns its own side effects
• Service detection moved to internal/automation/steps/ — cmd/link_services.go deleted

Test plan

• go test ./... — all 27 packages pass
• go vet ./... — clean
• gofmt -w . — clean
• Manual: pv link on a Laravel project verifies full pipeline runs
• Manual: pv link on a static/PHP project skips Laravel steps cleanly
• Manual: Set install_php_version: false in pv.yml, link project needing different PHP — verify warning shown