docs(runtime): document declared subpaths and Runtime Host composition

[ZxlDragonDoctor]

Problem

packages/runtime/README.md claimed "The package root barrel and the subpaths declared in package.json are supported public APIs", but packages/runtime/package.json exports has no "." entry — import('@maka/runtime') fails with ERR_PACKAGE_PATH_NOT_EXPORTED. The README also pointed composition at apps/desktop/src/main/main.ts, while the shared composition that constructs BackendRegistry and SessionManager lives in packages/runtime-host/src/server/execution-composition.ts.

Solution

• Document that only declared subpaths are public; remove the root-barrel claim.
• Add explicit import examples for SessionManager/BackendRegistry (@maka/runtime/session-manager) and AiSdkBackend (@maka/runtime/ai-sdk-backend).
• Point readers at Runtime Host composition; note Desktop is one host among clients.
• Align Extension rules wording with the same constraint.

Issue alignment

• No maintainer design note on docs(runtime): README advertises an unexported package root and stale composition entry point #5141; implements the issue's proposed scope (correct README, subpath examples, link shared composition).

Testing

• Docs-only; verified against packages/runtime/package.json exports and the Runtime Host import sites.

Addresses #5141

[me2seeks]

PR 5165 Review

结论

APPROVE
Docs-only：packages/runtime/README.md 的 root-barrel 误述已修正，补全部关键 subpath 的导入示例，并把 composition 指针换到真正的共享 composition 文件。所有 subpath 与导出逐一核对通过。

发现

[P3] 与 #5142、#5200 修同一缺陷、同一文件（issue #5141），三者不能同时合并

• 文件: packages/runtime/README.md
• 问题：本 PR、#5142、#5200 都只改 packages/runtime/README.md 的 "Public seam" 段，修的是同一个 root-barrel 误述（同一 issue #5141）。三个 PR 不能同时合并，后合者会因同段改动产生 diff 冲突。
• 建议：维护者三选一合并，其余关闭即可。本 PR 覆盖面最全（额外列出 sandbox/builtin-tools/shell-tools/runtime-kernel/test-only 的 subpath 并明确 BackendRegistry 导入来源），若求文档完整度可优先它。纯流程提示，不影响技术正确性。

验证

• package.json#exports 无 '.' 键 → root 确实不导出，原文档说法确为缺陷。
• PR 引用的 7 个 subpath（./session-manager、./ai-sdk-backend、./sandbox、./builtin-tools、./shell-tools、./runtime-kernel、./test-only/fake-backend）在 exports 中全部存在，无虚指。
• import { SessionManager, BackendRegistry } from '@maka/runtime/session-manager' 正确：两者都定义并导出自 src/session-manager.ts（SessionManager @898、BackendRegistry @768）。
• 新增相对链接指向的 packages/runtime-host/src/server/execution-composition.ts 存在并确实构造 SessionManager/BackendRegistry。

[me2seeks]

Thank you for the contribution and the additional entry-point documentation. After comparing the three fixes for #5141, we merged #5200, which covers the same documentation correction. Closing this PR as superseded; the choice reflects overlapping scope, not a correctness issue in this contribution.