Unify agentd and agent CLI workflow formats

agentd has its own Spec/Frontmatter types (workspace, toolchain, steps) while agent CLI parses different frontmatter (model, imports, tools, system_prompt). Need to converge on one format.

Current state:

• agent CLI: model, imports, tools, system_prompt, max_cost_cents, max_iterations
• agentd: workspace, toolchain, steps, model, provider, max_cost_cents, max_iterations, assertions

Goal: agentd should accept the same workflow .md format as agent CLI, then add container-specific fields (toolchain, workspace) on top.