lane-orders: grep saved test logs, never invent an unfilled placeholder

[erikdarlingdata]

Why

Wave E of #3898 ran 14 lanes at once. Two lane habits cost context or produced a wrong file:

• One lane saved its full test-suite log to a file and then Read the whole file: 47,000 characters in its context.
  The existing rule says not to paste full test output, but a saved log slipped past it.
• One lane's brief left the <CHANGELOG_BUFFER_DIR> placeholder unfilled. The lane made up a path inside the repo
  and committed its changelog entry there. The coordinator had to move it out.

What changes

.claude/skills/lane-orders/SKILL.md gets two rules:

• A test log saved to a file is still test output: grep it, never Read it.
• If a brief leaves a placeholder unfilled, the lane does not invent a value. It skips the step that needs it and
  says so in the PR body.

No code changes. No changelog entry, because this file is internal to how lanes work.

Test plan

• Read the diff. Both rules sit next to the rules they extend.
• No tests cover this file.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QQh8LczP1HGLXQYAx4oTZC