Dedupe builder README cache prose into Dockerfile comments

[zengsipei]

Closes #12

Implements ADR 0004 decision 2: the builder README's cache-persistence section no longer duplicates the Dockerfile's Rebuildable Cache wiring block.

Changes (devimage-build/README.md only, +5/−42)

• 缓存持久化 section: the volume-subdir → home-path mapping tree and the mkdir/ln -sf code blocks are removed; the section shrinks to loss-cost framing (Rebuildable Cache), a link to ADR 0003, and a pointer to the Dockerfile cache-wiring block as the sole owner (ADR 0004 decision 2).
• FAQ "如何添加新的缓存目录": the literal two-step Dockerfile recipe is replaced by "one-block edit in the Dockerfile's Rebuildable Cache wiring block, follow the existing entries" — says that it's a one-block edit without repeating the block's contents (per the issue's key-interface note).
• 用户配置 bullet: the (npm / pnpm / pip / poetry / vscode-extensions) tool enumeration — another copy that would rot — is collapsed into an anchor link to the cache section.

Acceptance criteria

• No mapping enumeration nor literal cache link command duplicates remain (grep -E 'cache-volumes|ln -sf|mkdir -p' on the README returns nothing)
• Remaining cache section links to ADR 0003 and points at the Dockerfile block
• scripts/check-release-contract.sh passes

Out of scope untouched: .devcontainer/ consumer README, Dockerfile wiring, no docs-vs-code sync checker.