docs(readme): v5 — model-driven focus, unit consistency, move cache section

[ranxianglei]

Summary

README refinement, 5 rounds of user feedback consolidated. Docs-only (no lib/ changes). EN + ZH kept consistent.

• Why ACP: "ACP hands all context-management authority to the model itself — best implementation to date." Two effects: saves ~2/3 tokens (a 1M-window model runs in the 200,000–300,000 range); ultra-long sessions without losing key content (500M-level context, 100K messages). Units unified; 37-bug-fixes footnote dropped (explained later).
• Proven at scale: one-line summary lead (500M-level context, p95 ~30%, avg cache hit >85% — average not per-session, explained in the cache section). Dropped Span / Model turns / Tokens-reclaimed rows. Sessions anonymized.
• How It Works: model 100% responsible; tools = compress/decompress/delete. Lifecycle as a 3-object mermaid state diagram (Raw ⇄ Compressed → Deleted). Compression strategy with the source priority list (from lib/prompts/system.ts: agent results, verbose command output, dead-end exploration, ...). Decompression + Deletion strategies (deletion irrecoverable, replaces forced GC).
• Impact on Prompt Caching: ~87% hit rate; beats traditional compression (which acts at 80–90% + forces full re-hit); ACP keeps context ~30% vs traditional 50–80%. Moved to right after How It Works.

Verification

• Docs-only — no lib/ touched, no build/typecheck/test impact.
• Language cross-links present in both READMEs ([English](./README.md) | [中文](./README.zh-CN.md)).
• No real project/session names leak (anonymized to Session 1 / Session 2).
• Devlog: devlog/2026-06-27_readme-v3/.

Non-goals

• No version bump (1.4.0 is already on npm; a follow-up release will pick this up if desired).
• LICENSE unchanged (AGPL-3.0-or-later) — MIT relicense is not legally possible on an AGPL fork.

Branch pushed via the GitHub git database API because direct git push to github.com:443 times out in this environment (GnuTLS); api.github.com is reachable. Reviewed against ranxianglei/opencode-acp master e981bb51.