refactor(instructions): centralize AGENTS.md core and gate session context injection

[hapo-nghialuu]

Summary

Wave 1 of the instructions refactor: shrink the always-on instruction layer that every user loads each session, and make the machine actually verify what ships.

• AGENTS.md is now the canonical shared instruction file. CLAUDE.md becomes a thin Claude-specific wrapper importing it via @AGENTS.md. always-on của user Claude: ~130 → ~64 dòng (AGENTS.md 39 + CLAUDE.md ~25).
• The dynamic rules hook injects once per session instead of on nearly every prompt. The old 5-minute cooldown never held in real sessions. The injected block also drops from ~25 lines of generic advice (YAGNI/KISS/DRY, skill routing, modularization) to paths + docs.maxLoc + Language.
• Kept the project-specific gotchas across all three runtimes: Completion Criteria / ## Evidence as source of truth, the NO_TESTS rule, skill venv paths for macOS and Windows, edit-skills-in-project, <!-- cafekit:lang --> marker, addressing canary.

Defects found by independent review and fixed in this branch

An independent read-only review of the first implementation found five blocking defects. All five were reproduced by execution and fixed:

1. Missing runtime.json injected defaults anyway — including a Plans/Docs path resolved from a possibly stale PROJECT_ROOT, which could point at a different project. Now silent. payload.cwd is authoritative; PROJECT_ROOT is a fallback only.
2. OpenCode never got its language patched — post-install.js built instruction targets for claude and codex only, so --lang vi left OpenCode in English.
3. Codex and OpenCode lost the venv and skill-location rules during the template rewrite.
4. Combined install duplicated the shared core three times in root AGENTS.md. New bin/lib/instruction-blocks.js manages a <!-- CAFEKIT CORE START --> / <!-- CAFEKIT CORE END --> block; later installers detect it and append only their runtime-specific block. Idempotent across re-runs.
5. The self-test suite proved nothing about shipped output. Mutation testing showed that deleting the shared gotchas, or breaking the managed-block markers, still produced PASS: 211. Assertions now run against real installs into temp directories.

Verification

Each item verified by installing into mktemp -d directories rather than reading source:

Check	Result
Missing runtime.json → injection	0 bytes (Claude and Codex)
OpenCode --lang vi	Vietnamese present, English absent
venv + skill rule per platform	present in claude, codex, opencode output
Combined install core markers	1, still 1 after re-running the installer
Mutation test (venv removed from core)	suite now exits 1
Test suite	225 passing, up from 211

Known issue, not fixed here

node bin/install.js --dry-run writes into the repository it runs from — it modified packages/spec/.gitignore and created packages/spec/AGENTS.md and packages/spec/opencode.json. Pre-existing installer bug, out of scope for this wave, worth a separate fix.

Test plan

• npm test in packages/spec (225 tests)
• Fresh install per platform: claude, codex, opencode
• Combined install, then re-run to confirm no duplication
• Non-English install (--lang vi) on each platform
• Confirm an existing project's customizations survive an upgrade over this version