[Change] Sprint planning (capacity and commitment) support

[djm81]

Why

SpecFact CLI supports sprint/release assignment and story points at the backlog-item level, but there is no first-class support for sprint capacity, commitment vs capacity comparison, or a CLI/export view that shows sprint-level summary. Teams must manually sum story points and compare to capacity outside SpecFact.

What Changes

• Introduce lightweight "sprint capacity" (config or file per project: capacity in story points per sprint).
• When exporting or listing backlog items filtered by sprint, compute total story points and compare to capacity.
• Add specfact backlog sprint-summary: sprint id, total committed points, capacity, difference (over/under).
• E2 (plan): Optional sprint_goal support; show alignment hints. Include risk rollup in sprint summary. Add "DoR coverage" summary via Policy Engine. Acceptance: Sprint summary includes capacity, committed, risk, top blockers, DoR pass rate.

Acceptance Criteria

• Sprint capacity config; commitment sum by sprint; over/under output.
• Sprint summary includes: capacity, committed, risk (when available), top blockers, DoR pass rate (when Policy Engine available).
• Documentation updated for sprint planning.

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OpenSpec Change Proposal: sprint-planning-capacity-commitment-support