[IR-P7][WS6] Finalize migration guide and upgrade checklist for compatibility window

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Problem statement

Phase 7 deliverable 2 needs a dedicated issue to publish migration guidance for the public release compatibility window.

Intended invariant

Migration docs explicitly state that deprecated methods remain supported through the Phase 7 window and are removed in v0.13.0.

Scope boundaries

• Implements only this Phase 7 deliverable
• Covers migration guide + upgrade checklist updates
• Does not remove deprecated code paths

Deliverable checklist

• Implementation complete for this deliverable
• Tests/docs checks updated and passing
• Migration impact recorded with rationale
• Docs impact recorded with required updates landed
• Evidence linked for roadmap phase gate

Validation plan

Verify docs against at least one realistic upgrade flow and attach evidence.

Parent epic

#100

Roadmap reference

Roadmap reference: docs/plans/2026-06-19-001-ir-first-roadmap.md (Phase 7)

Migration impact

minor - clarifies upgrade behavior and removal timeline (v0.13.0).

Documentation impact

new-docs - migration/upgrade documentation is the core deliverable.

Sync status

Synced with roadmap: 2026-06-19