Step 2 of #874: a repo- and tech-stack-scoped experience library #946

Step 2 of #874 only. Merging the pull request that fixes this issue does not close the RFC: #874 still carries Step 3, and no Fixes #874 belongs anywhere.

What

An offline experience library, keyed by repository and tech stack, kept separate from the root LESSONS.md. The host reads it; nothing on the turn path changes.

Acceptance (all of it holds at cd7b314d, the tip of #941)

1. Lessons are JSON files under core/dev/experience/repos/<owner>/<name>/ and core/dev/experience/stacks/<techStack>/. Seeds: repos/ThinkFlowLab/FlyBot/stale-base.json, repos/ThinkFlowLab/FlyBot/one-conclusion.json, stacks/node-host/generated-gates.json.
2. lookupExperience (core/src/host/experience.js:164) reads exactly one scope: a repo lookup opens only that repo's directory (:225), a tech-stack lookup returns entries whose repo is null (:249), and a call with neither key returns an empty list. There is no model call on this path.
3. rt.harnessExperience (core/src/host/harness.js:228, installed at :236) is the host-facing reader; harnessPro (:176) still names CI, core/dev/eval and rsi-baseline.
4. node core/dev/experience/check.mjs validates the tree (checkExperience, core/src/host/experience.js:264). Its add writes one new file and refuses to overwrite; it does not edit a harness config.
5. The Lite / Medium / Heavy budgets are unchanged and applied stays false; the tier harnesses (core/src/host/harness.js:88) route exactly as Step 1 left them.
6. The read API lives in core/src/host/experience.js; the JSON library lives in core/dev/experience/, which the npm package does not ship. So a packaged install's default read returns no entries. Test: core/test/experience.test.js.
7. patches/ds-bot/files/ carries byte-identical copies of every added or modified core file, and node scripts/ds-bot-patches.mjs check passes.

Verification

cd core && TZ=UTC npm test
node core/dev/experience/check.mjs
node scripts/ds-bot-patches.mjs check

Out of scope

Step 3. Option D (an online self-modifying harness). Injecting lessons into the prompt. Changing the merge gate or enqueue-guard. Replacing CI, core/dev/eval or rsi-baseline. Closing #874.

Notes

• Implementation: Add a repo-scoped experience library for harness step 2 #941, 18 files, +1544/-1, branch cursor/experience-library-step-2.
• No changelog fragment: maintainer and agent tooling, not a user-visible product change (AGENTS.md 29), same as chore(ci): report a review round that calls someone other than the declared reviewer (#911) #912 / Step 1 of #874: three loaded harness tiers (harness:lite/medium/heavy), per-tier budgets, offline harnessPro, and the usage.harness column #937.