docs: plan sidebar clip instrument management

[britle20]

Summary

• Adds a feature spec for sidebar clip and instrument management.
• Documents serializable clip-level pitched instrument membership.
• Updates the active issue queue to remove closed issue 26 and add issue 30 after the sampler sustain work.

Changes

• Added docs/features/12-sidebar-clip-instrument-management.md.
• Updated PLANS.md Active Issue Order and planned milestones.
• Updated docs/data-model.md with M1 clip collection and sidebar membership rules.
• Updated docs/ui-design.md with sidebar add/delete and instrument picker guidance.
• Updated docs/testing.md with sidebar model transformation risk areas.

Testing

• git diff --check
• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Notes / Limitations

• This is planning only; no production sidebar implementation is included.
• The implementation issue is Sidebar clip and instrument management #30.
• References Sidebar clip and instrument management #30.