Add top-level goal frontmatter support and map it to Codex --goal

[Copilot]

This change adds goal as a first-class top-level frontmatter field (instead of nesting under engine) and wires it into Codex invocation so workflow intent can be passed explicitly as a CLI goal while the markdown/problem body remains the instruction source.

• Frontmatter contract

  • Added top-level goal to typed config (FrontmatterConfig) and workflow schema (main_workflow_schema.json).
  • Documented goal in frontmatter and engines reference docs.

• Compilation/runtime propagation

  • Added Goal to WorkflowData.
  • Populated WorkflowData.Goal from parsed frontmatter during workflow build (resolveWorkflowGoal), with trimmed raw fallback.

• Codex execution mapping

  • Updated Codex command generation to append:
    • ${GH_AW_GOAL:+ --goal "$GH_AW_GOAL"}
  • Injects GH_AW_GOAL env only when a top-level goal is set, avoiding empty/implicit goal behavior.

• Coverage updates

  • Extended parsing and Codex execution tests for goal-present and goal-absent paths.
  • Updated Codex wasm golden output to include the new command shape.

---
engine: codex
goal: "Fix flaky tests in pkg/workflow"
---
# markdown problem/instructions

Codex execution now includes:

codex exec ... ${GH_AW_GOAL:+ --goal "$GH_AW_GOAL"} ... --prompt-file /tmp/gh-aw/aw-prompts/prompt.txt