[Doc] Clarify path-vs-body convention in /issue Step 4 and Step 6 briefs

Parent: #275 (audit item 3)

Background

Per the audit posted on #275 (item 3), the dispatch briefs in .claude/skills/issue/SKILL.md Steps 4 (implementer) and 6 (experimenter) say "The plan" without specifying body-vs-path. In practice the orchestrator passes the path .claude/plans/issue-<N>.md; the subagent is expected to Read the file before acting.

This is the right default (1400-line plans inlined into a prompt waste context, can hit token limits, and stale across worktree edits). But the convention is not documented, so an adversarially-loaded subagent might guess at plan contents instead of reading the file.

Proposed fix

Add a 1-line clarification to both briefs:

Plan handoff convention: the brief includes the PATH to the cached plan (.claude/plans/issue-<N>.md), NOT the body. Read the file before acting; do NOT infer plan content from the issue body or comment markers.

That's it — pure doc PR, no behaviour change.

Acceptance criteria

• .claude/skills/issue/SKILL.md Step 4 brief includes the path-vs-body clarification.
• Same for Step 6 brief.
• No code change, no behaviour change.