fix(dag): harden workflow spec handoffs

[LeXwDeX]

Summary

• move start, extend, and replan graph payloads from nested tool arguments into reusable YAML spec files
• derive protocol version, admission state, and brief fingerprint at the workflow boundary
• render structured template inputs as indented JSON instead of [object Object]
• update orchestration guidance and regression coverage

Root cause

Weak-model accuracy triggered malformed nested arguments, but the structural cause was a shallow capability exposed through a large writable schema. The second retry could also submit a server-owned CONSUMED state. Separately, template interpolation used JavaScript string coercion for structured values.

Verification

• packages/opencode: bun test test/dag --timeout 30000 — 263 pass
• packages/opencode: bun typecheck
• packages/core: bun typecheck
• packages/core: bun test test/plugin/command.test.ts --timeout 30000 — 16 pass

No HTTP API shape changed, so SDK regeneration is not required.