Complete sequence diagrams across the canonical pipeline #280

Goal

Complete sequence-diagram support across Sketchi's canonical diagram pipeline and use it as the reference path for future explicitly supported diagram families.

Current baseline

• diagram-core currently registers flowchart and mindmap as typed families.
• Sequence diagrams now have an explicit Code Mode contract, native renderer, Excalidraw output, and public build endpoint.
• Natural-language generation, CLI dispatch, maintained scenarios, and Storybook do not yet treat sequence as a canonical family.
• Unsupported families fail explicitly; there is no generic fallback to preserve.

Scope

• Establish one typed sequence contract and validation boundary shared by the relevant entry points.
• Integrate sequence with diagram-core, generation, CLI dispatch, scenarios/evals, and Storybook where those surfaces advertise diagram families.
• Preserve sequence-specific participant, message-ordering, and lifeline semantics rather than mapping it to a flowchart.
• Update the diagram-type generator or extension guidance so a future family cannot omit required pipeline stages silently.

Non-goals

• Creating speculative issues for organization, ER, Gantt, swimlane, or other unselected families.
• Reintroducing the retired generic diagram-type fallback.

Acceptance criteria

• Sequence is represented by one canonical typed contract rather than parallel surface-specific shapes.
• Supported generation and CLI entry points can request sequence diagrams explicitly.
• Deterministic renderer, Excalidraw validation, scenario/eval, API, CLI, and Storybook coverage is present where applicable.
• Unsupported families continue to fail explicitly and are not advertised.
• The documented extension path identifies every required integration point for the next selected family.

Current touchpoints

• packages/diagram/agent/src/lib/code-mode-contract.ts
• packages/diagram/renderer/src/sequence.ts
• packages/diagram/core/src/diagram-types.ts
• packages/diagram/generation/
• packages/diagram/scenarios/
• packages/diagram/ui/
• apps/playground/src/routes/api/v1/sequences/build.ts
• apps/cli/src/builder.ts
• tools/sketchi-generators/src/generators/diagram-type/

This replaces the still-actionable part of #69 after native Code Mode sequence support landed in #277. Add other families only when product demand or eval evidence selects them.