feat: add canvas dashboard CLI commands

[forestileao]

Summary

• add superplane canvases dashboard get <canvas> for exporting dashboard YAML/JSON
• add superplane canvases dashboard update <canvas> --file dashboard.yaml with create/apply aliases for first-time creation or replacement
• keep the dashboard skill mounted in Anthropic default session resources
• add focused CLI dashboard resource and command tests

Refs #4927

Tests

• make format.go
• go test ./pkg/cli/commands/canvases ./pkg/cli/commands/canvases/models
• go test ./pkg/agents/anthropic -run TestDefaultResourceSourcesForSkillsBaseURL
• make lint && make check.build.app (lint passes; check.build.app fails on existing generated-proto drift: CapabilityDefinition.ExampleOutput, CapabilityDefinition.ExampleData, and CanvasRun.VersionId are referenced by current code but missing from generated protos)
• go build cmd/cli/main.go (fails on existing generated-client drift: IntegrationsCapabilityDefinition.ExampleOutput and ExampleData are referenced by current index commands but missing from generated client)

[superplanehq-integration]

👋 Commands for maintainers:

• /sp start - Start an ephemeral machine (takes ~30s)
• /sp stop - Stop a running machine (auto-executed on pr close)

[cursor]

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