feat: support trigger and tag plans

[vecyang1]

Summary

• compile validated trigger config into upstream GTM commands
• bind created tags to one or more reviewed trigger IDs
• preserve dry-run, execute, inventory, version and double-publish gates
• document and fake-E2E the two-plan trigger→inventory→tag workflow

Production proof

• used the new flow to publish Carradio GTM version 6
• live diff added only DLVs 44/45/46, custom-event trigger 47 and GA4 event tag 48
• Tag Assistant observed a real article_product_click and tag 48 firing once
• no cross-domain linker was added

Verification

• go test ./...
• go vet ./...
• go mod verify
• fake GTM E2E
• go build ./...
• gitleaks and git diff --check
• independent code review approved with no findings