Forbid editing accepted decision bodies during auto-audit

[jimutt]

Summary

• Adds an explicit "Hard rule" to dld-audit-auto: the narrative body of an accepted decision is immutable. The audit may only touch housekeeping frontmatter (references, amends, superseded_by) and @decision annotations in code.
• Removes the "minor update / refine the decision record" path from the unreferenced-code-change handling. If new behavior or rationale is detected, the audit must create a new decision (linked via supersedes/amends) instead of editing the existing one.
• Mirrored in both skills/dld-audit-auto/SKILL.md (Tessl) and .claude/skills/dld-audit-auto/SKILL.md (Claude Code) per the dual-directory convention.

Test plan

• Read both SKILL.md copies and confirm the hard-rule wording is consistent