Instant placeholder PR comment + shared preview comment-script extraction (PLAN item 6)

[mmcky]

#92 Phase 2: instant first signal on preview PRs, plus the shared comment-script extraction (PLAN item 6).

• Instant placeholder PR comment — no container, no checkout (REST changed-files) — with deterministic pr-N links; today both actions comment only after a successful deploy (preview-netlify/action.yml:101, preview-cloudflare/action.yml:148)
• Extract the shared comment script: the two github-script bodies (preview-netlify/action.yml:111-186 vs preview-cloudflare/action.yml:159-242) now differ by ~12 lines (a deploymentUrl const, two title lines, two marker lines, a 7-line "This deployment" block) against PLAN item 6's recorded "only 2 lines differ"

The Cloudflare stable-alias half of the original bundle shipped in v0.11.1 (#131). The extraction also serves the Workers path in #145, whose change-detection and comment machinery is identical; the placeholder comment should emit the Workers alias URL template once QuantEcon/infrastructure#4 lands.