[P0] Introduce canonical compiled job contract into agenc-core executor boundary

[signerless]

Summary

Introduce a canonical compiled job contract and executor boundary in agenc-core so runtime execution is driven by a versioned structured plan instead of raw task handler/context orchestration.

This is a direct blocker for #1557.

Current Evidence

The current task executor path is built around raw task execution primitives, not a marketplace compiled-job contract:

• /Users/tetsuoarena/agenc-umbrella/agenc-core/runtime/src/task/executor.ts
• /Users/tetsuoarena/agenc-umbrella/agenc-core/runtime/src/task/types.ts

I did not find a canonical runtime contract that carries:

• job_type
• risk_tier
• allowed_tools
• allowed_domains
• memory_scope
• write_scope
• network_policy
• compiled_plan_hash
• compiler_version
• policy_version

Goal

Define and wire a canonical compiled job contract into runtime execution so the executor receives only:

• compiled plan
• resolved policy output
• scoped runtime credentials / tokens
• bounded budgets and approval requirements

Raw freeform task text must never be the permissions source.

Scope

Primary repo:

• agenc-core

Expected architectural work:

• define the runtime-facing compiled job type
• map marketplace job execution onto that type
• persist plan and policy audit fields per run
• fail closed on unknown job type / policy state / tool scope mismatch

Acceptance Criteria

• executable marketplace jobs run through a canonical compiled job contract
• runtime receives a structured, versioned plan instead of freeform permissions
• compiled_plan_hash, compiler_version, and policy_version are stored with each run
• runtime rejects unknown or incomplete compiled plans
• executor cannot broaden scope from raw prompt text at runtime

Notes

This is the core technical boundary change in #1557. Without it, the epic remains architecture only.