Complete OpenClaw agent artifact co-location

> [!IMPORTANT]
> Reserved for @cv. Do not self-assign this issue. Do not open an implementation PR unless @cv requests help.

Parent: #7691

## Problem

The OpenClaw manifest records Dockerfiles, the start script, policy, and plugin as legacy paths outside `agents/openclaw/`.

## Scope

- Complete the existing Phase 3 artifact move.
- Co-locate OpenClaw-owned runtime assets under `agents/openclaw/`.
- Update build, package, validation, and test paths.

## Acceptance criteria

- The manifest has no `_legacy_paths` block.
- OpenClaw-owned assets have one directory owner.
- Package and build-context contracts pass.
- OpenClaw runtime behavior remains unchanged.