docs(runtime): correct public imports and composition entry point

[MasamiYui]

Summary

The Runtime README described a supported package-root import even though @maka/runtime has no root export, and pointed contributors to Desktop main for Runtime composition.

This corrects both root-barrel references, adds explicit SessionManager and AiSdkBackend subpath import examples, and links the shared Runtime Host execution composition. The change is limited to packages/runtime/README.md; runtime behavior is unchanged.

Fixes #5141

Verification

Verified on macOS 26.6.2 / Node.js v23.7.0, against base f09119884 and this branch:

• Actual import('@maka/runtime') rejects with ERR_PACKAGE_PATH_NOT_EXPORTED, confirming the existing documentation defect.
• Both README example subpaths match the package export map and resolve to their declared targets; both named classes exist in the corresponding source modules.
• Both added relative links resolve to existing files.
• The linked Runtime Host composition constructs SessionManager and BackendRegistry.
• git diff --check HEAD^ HEAD passes.
• Inspected the actual GitHub Markdown rendering before and after the change.

The subpath checks cover resolution and source declarations, not execution of compiled Runtime modules. Full lint, formatter, typecheck, build, and workspace test suites were not run for this Markdown-only change.

Before and after screenshots — actual GitHub Markdown preview

Before — base README

After — updated README

Screenshots are stored on a separate evidence branch in the fork and are not part of this PR's file changes.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex inspected the package exports and composition, edited the README, ran the focused verification, and captured the screenshots. The commit includes Generated-by: OpenAI Codex.

Automated PR submission by OpenAI Codex on behalf of @MasamiYui, the human contributor of record.

Checklist

• Tests cover the change and fail without it — N/A for runtime regression tests; this changes documentation only.
• Lint, format, typecheck and the affected suites pass locally — full suites not run; focused documentation checks are listed above.

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No