docs: Phase P plan — sc-lint 0.5.0 dual-consumer upgrade qualification

[randlee]

Summary

• Adds the authoritative Phase P plan: qualifying a released sc-lint 0.5.0 consumer contract against both sc-compose and atm-core before either repo converts.
• Adds sprint records P.1 (dual-reference qualification), P.2 (sc-compose conversion), P.3 (atm-core acceptance) — all currently status: planned, no implementation started.
• Documents the ADR-0016/ADR-0017 amendment requirement and the reserved-recipe migration gate blocking P.2/P.3 until P.1 passes.

Status

Planning-only PR for review. No implementation sprints have started; the plan is blocked on an upstream released sc-lint 0.5.0 artifact.

Test plan

• Review plan structure, requirement/ADR traceability table, and sequencing for correctness
• Confirm boundary-rule compliance section matches CLAUDE.md rules
• Confirm sprint docs (P.1–P.3) are consumable as-is by implementers

🤖 Generated with Claude Code