fix(dag): harden workflow startup controls

[LeXwDeX]

Summary

• add global /dag-cancel with deterministic current-session targeting
• remove workflow-level model overrides and prefer dag.jsonc tiers over agent defaults
• ask parent-session QA and avoid workflow creation when no model can be resolved
• update orchestration guidance and retain legacy persisted-model compatibility

Validation

• bun test test/dag/workflow-tool.test.ts test/dag/spawn-completion.test.ts test/dag/dag-config.test.ts
• bun test test/feature-plugins/dag-inspector.test.tsx
• bun typecheck in packages/core, packages/opencode, and packages/tui
• pre-commit lint and full monorepo typecheck
• pre-push full monorepo typecheck