docs: surface htmlbin-publish skills.sh install across all agent surfaces

[utsengar]

Summary

Threads the one-command htmlbin-publish install (shipped in
utsengar/htmlbin-cli#11)
through every public surface an agent or human might check first.

npx skills add https://github.com/utsengar/htmlbin-cli --skill htmlbin-publish

Surfaces touched

• Landing (src/views/landing.ts) — third "skill" tab in the prompt block alongside agent / cli. Prompt-cue updated.
• /api/onboard (src/onboard.ts) — new skill: block in the JSON descriptor and an "Install as an agent skill" section in the markdown variant. buildOnboardJson() and buildOnboardText() stay in sync per the existing convention.
• Worker-served reference skill (src/skill.ts + skills/htmlbin/SKILL.md mirror) — "Installable companion" section right after Source of truth: /api/onboard, framing the two skills as complementary (this one = comprehensive reference; htmlbin-publish = focused workflow).
• agent-card.json + llms.txt (src/discoverability.ts) — adds skill: field with install command, repo, supported runtimes; install paragraph in llms.txt.
• README.md — top-of-file "For agents — one-command install" callout.
• CLAUDE.md — new "Two skills, one contract" section enumerating every cross-reference so future PRs that touch the skill name / install URL / repo location sweep all of them in one go.

No runtime behavior changes — pure docs / copy surfaces. Typecheck passes (node node_modules/typescript/bin/tsc --noEmit clean).

Test plan

• Wait for CI's sticky preview URL comment.
• On the preview URL, confirm the new skill tab renders in the prompt block, copies the install command cleanly, and the prompt-cue reads correctly.
• curl -s <preview>/api/onboard | jq .skill returns the install command and supported runtimes.
• curl -s -H "Accept: text/markdown" <preview>/api/onboard | grep -A2 "Install as an agent skill" returns the new section.
• curl -s <preview>/.well-known/agent-card.json | jq .skill returns the new block.
• curl -s <preview>/.well-known/agent-skills/htmlbin/SKILL.md | grep -A2 "Installable companion" returns the new section.
• curl -s <preview>/llms.txt | grep -A3 "Install as an agent skill" shows the install command.
• Mobile-check the landing at 360px (the third tab shouldn't push the chrome wider).

🤖 Generated with Claude Code

[github-actions]

🚀 Cloudflare preview

URL: https://19dd8d6f-htmlbin.utkarsh2012.workers.dev

Built from commit b63f7fe1564800508567e86d2689b9a28f4c7012.

Note: bindings (D1, KV, AI) are shared with production.
Avoid destructive PR tests on real data.