Render AGENT.md template parameters from work item state

[devzeebo]

Summary

Task Agent AGENT.md files declare template.parameters and use {{token}} placeholders in the prompt body. Today the parser validates that tokens are declared, but nothing substitutes values from work item state at run time.

Current behavior

• agent-parser.ts validates {{tokens}} against template.parameters at parse time
• ? suffix marks optional parameters in declarations only — no runtime enforcement
• verifyIsTaskAgentState checks workingDir, instructions, engineName, and sessionId — not template parameters
• Engines pass raw agent.promptBody to the model with literal {{...}} still in the text (cursor-engine.ts, prompt.ts)

Desired behavior

1. Render — substitute {{parameterName}} in promptBody from work item state before the engine builds the final prompt
2. Validate — parameters without a ? suffix are required in state; missing required parameters should fail before engine execution
3. Optional — parameters declared with ? may be omitted from state

Docs

Usage guide describes this as planned behavior: using-task-agents.md

Out of scope

Workflow child enrichment (instructions, engineName, template params on task steps) — that remains the job of step decorators, not the workflow framework.