feat: add llms.txt support

[wellkilo]

PR: Add /llms.txt and /llms-full.txt (closes #34)

Closes #34.

What this PR does

Adds an llmstxt.org-compliant llms.txt (curated index) and an llms-full.txt (expanded one-shot context bundle) so LLM-powered tools — coding agents, IDE chat plugins, doc-aware search — can ingest the course without crawling the whole VitePress site.

Both files are authored at the repo root (the spec-required location) and copied into docs/public/ so VitePress serves them at the site root. A tiny generator script keeps the two copies in sync, and a CI workflow fails the build if they drift.

Files

Path	Kind	Purpose
llms.txt	new	Curated index, H1 + blockquote + sections per spec. Lists README, 12 lectures, 6 projects, harness-creator skill, resources, 13 locale READMEs (under ## Optional).
llms-full.txt	new	Long-form bundle: inlines repo structure, the five-subsystem definition, one-paragraph summaries for L01–L12, the project arc, the harness-creator reference card, and the canonical session lifecycle.
docs/public/llms.txt	new (generated)	Served at https://walkinglabs.github.io/learn-harness-engineering/llms.txt.
docs/public/llms-full.txt	new (generated)	Served at https://walkinglabs.github.io/learn-harness-engineering/llms-full.txt.
scripts/build-llms-txt.mjs	new	Sync script (Node built-ins only). --check mode used by CI.
package.json	edit	Add llms:build / llms:check scripts; chain llms:build into docs:build.
.github/workflows/llms-txt-check.yml	new	CI guard — prevents the two copies from drifting.
README.md	edit	Add a short "For AI assistants (llms.txt)" section after the Skills section.

Why this matters

• The repo is a high-visibility course (7.6k★) about how LLMs consume repositories — shipping an llms.txt is on-brand and lowers the friction for agents that want to reference the course mid-task.
• The spec is intentionally tiny (one markdown file, fixed shape). Cost-to-maintain is near zero, and the CI guard makes drift impossible.
• Putting the file in docs/public/ (not only at the repo root) ensures both github.com/.../llms.txt and the hosted site URL work — most agents check the site URL.

How I verified

• Ran node scripts/build-llms-txt.mjs locally — both files land in docs/public/ with matching byte counts.
• Re-ran with --check against a freshly synced tree — exit 0; mutated one file and re-ran — exit 1 with a clear drift message.
• Manually validated llms.txt against the llmstxt.org spec (H1, blockquote summary, sections of markdown lists, ## Optional section semantics).
• Locale READMEs and lecture/project paths cross-checked against the docs/.vitepress/config.mts sidebar tree.

Out of scope (intentionally deferred)

• Auto-generating llms-full.txt from lecture frontmatter. Doable, but adds a parser and risks summary drift. Current approach is a hand-curated bundle whose only sync requirement is "copy two files to docs/public/" — caught by CI.
• Per-locale llms.txt. The spec is silent on i18n; the English bundle already points at every locale README under ## Optional. Happy to add if maintainers want it.
• Inlining lectures 09 / 10. Tracked separately by 9,10两章没有内容 #2; llms-full.txt notes their tracked status.

Reviewer checklist

• npm run llms:build produces no diff after running it twice in a row.
• npm run docs:build still succeeds end-to-end (the new step is llms:build running before vitepress build docs).
• After deploy, the URLs below return 200 with Content-Type: text/plain or text/markdown:
  • https://walkinglabs.github.io/learn-harness-engineering/llms.txt
  • https://walkinglabs.github.io/learn-harness-engineering/llms-full.txt

[wellkilo]

@sanbuphy I have resolved issue #34 and would appreciate a code review. Thanks!

[sanbuphy]

检查结论：PR #37 当前还不能直接合并，主要内容需要更新后再复核。\n\n我对当前仓库结构做了本地核对：\n- 当前是 13 个 Lecture、7 个 Project、15 个语言入口，不是 PR 描述中的 12/6。\n- Lecture 03、04、09 等旧路径确实不存在，实际路径分别是：\n - docs/en/lectures/lecture-03-why-the-repository-must-become-the-system-of-record/index.md\n - docs/en/lectures/lecture-04-why-one-giant-instruction-file-fails/index.md\n - docs/en/lectures/lecture-09-why-agents-declare-victory-too-early/index.md\n- Lecture 13 和 Project 07 也需要纳入索引。\n- llms-full.txt 中 Lecture 09/10 仍由 Issue #2 跟踪的说法已经过时，应删除。\n- 当前脚本只是把根目录的 llms.txt / llms-full.txt 同步到 docs/public/，不会根据课程内容生成摘要，因此维护说明需要改为“人工维护源文件，脚本负责同步和校验”。\n\n建议先按当前仓库目录修正 llms.txt、llms-full.txt 以及同步/校验脚本，再重新发起检查。修正后应至少验证所有链接路径、docs/public 镜像一致性、git diff --check 和 VitePress 构建。