Document ACM compression philosophy and cadence

[KorenKrita]

目的

记录 PR14 后续讨论形成的 ACM 设计结论，作为实现“道、术、度”一体化的独立设计依据。

核心结论

• Compression is intelligence.
• Compression is continuous; folding is batched.
• Cadence follows durable representation gain, not action count.
• Agent 默认可以自主调用 checkpoint、timeline 和 travel；只有用户明确表示接下来不要 travel 时才暂停 travel。
• 保留七槽 handoff 外壳，但允许 State 表达进行中的已知、未知、假设、归因、hot set 和判别动作。
• 用 uncertainty fidelity、compression seam 和 continuation fidelity 替换保守的 active-uncertainty / closed-boundary / cold-start 语义。
• 同时约束 context sediment 与 compression thrash，不规定固定调用流程或全局次数。

范围

本 PR 只增加设计文档，不修改运行时代码、提示词或测试。后续实现会另行提交。

验证

• git diff --check
• outgoing range 仅包含 docs/acm-compression-design.md

[KorenKrita]

Superseded by the compression-as-intelligence redesign now on main (see commits from 4a7a1 range onward: CORE.md carries the dao/du doctrine, TOOL-CONTRACTS.md carries the shu mechanics, and the cadence band — sediment vs. thrash with a ~1/3 working-budget cruise preference — is implemented in the runtime nudges). The philosophy this PR documented is now canonical in skills/context-management/CORE.md.