feat(agent): expose native sequence diagrams in Code Mode

[anandpant]

Summary

• add a deterministic native sequence renderer with participant headers, lifelines, and chronological messages
• expose buildSequenceDiagram through the Code Mode contract, runtime, MCP namespace, HTTP API, docs, and all three skills
• cover contract validation, artifact export, invalid references/self messages, MCP registration, and HTTP routing

Validation

• pnpm nx run-many -t typecheck,build
• pnpm nx run-many -t test --exclude=sketchi-cli
• pnpm lint
• pnpm nx build-storybook diagram-ui

Closes #236

[anandpant]

• feat(agent): expose native sequence diagrams in Code Mode #277 👈 (View in Graphite)
• main

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[github-actions]

Sketchi Web Preview

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• Project: web
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• Preview Worker: sketchi-web-pr-277
• Commit: e1d6d0dbf372
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Preview Worker cleanup has completed.

[github-actions]

Sketchi Eval Harness Preview

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Preview Worker cleanup has completed.

[github-actions]

Sketchi Icons Preview

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• Project: icons
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• Route policy: icons.sketchi.app product surface
• Preview Worker: sketchi-icons-pr-277
• Commit: e1d6d0dbf372
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Preview Worker cleanup has completed.

[github-actions]

Sketchi Excalidraw Workspace Preview

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• Project: excalidraw
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• Commit: e1d6d0dbf372
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Preview Worker cleanup has completed.

[github-actions]

Sketchi Playground / Studio Preview

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• Commit: e1d6d0dbf372
• Workflow run: https://github.com/shpitdev/sketchi/actions/runs/30046047963

Preview Worker cleanup has completed.