docs(plans): add markdown link/anchor integrity implementation plan

Plan for spec items 1-2 (GitHub-slug anchors + link/anchor resolution
surfaced via fitness_report); items 3-4 explicitly deferred to their own
plans. 6 tasks, 0 HIGH risk, 1 CROSS boundary (calm-core -> calm-server).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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