docs: gstack compact design doc (tabled pending Anthropic API)

[garrytan]

Summary

• Adds docs/designs/GCOMPACTION.md as a preserved design artifact for gstack compact (PostToolUse compactor + Haiku verifier + B-series benchmark).
• Status: TABLED. Research on 2026-04-17 confirmed Claude Code's PostToolUse cannot replace non-MCP tool output today, and Read/Grep/Glob don't fire hooks at all. Blocks on Anthropic issue #36843.
• No code, no hook, no CHANGELOG entry, no VERSION bump. The doc exists so a future sprint can un-block quickly when/if the API ships.

What's in the doc

• Status + evidence (docs quote, issue #36843, RTK source review, tokenjuice source review)
• Un-tabling checklist (5 steps to resume when the API lands)
• 15 eng-review decisions (scope, architecture, code quality, testing, perf)
• Full architecture: JSON envelope, 4 primitives, verifier tiers, tee mode, B-series testbench
• 30-case P-series + G-series + V-series + B-series test plans
• Codex review report (BLOCK verdict) + research findings
• Office hours diagnostic + competitive deep-dive (RTK, tokenjuice, caveman, etc.)

Test plan

• No code changes, no tests needed.
• Doc renders correctly on GitHub.

🤖 Generated with Claude Code

[github-actions]

E2E Evals: ✅ PASS

0/0 tests passed | $0 total cost | 12 parallel runners

Suite	Result	Status	Cost

---

12x ubicloud-standard-2 (Docker: pre-baked toolchain + deps) | wall clock ≈ slowest suite