Workflow determinism: snapshot or generation-pin the Agent definition per stage

### Background

#87 (landing in #169) snapshots the **AgentWorkflow** definition onto `AgentWorkflowRun.status` at init — the stage list, each stage's `agentRef`/`instructions`/`execution`, the `guide`, and the workflow-level `params` — and executes from that snapshot. So editing the **AgentWorkflow** mid-run cannot change stages already planned.

### Gap

We snapshot the workflow, but **not the Agent definitions** the stages reference. The stage snapshot records only `agentRef` (the name). Each stage's AgentRun is created *as the workflow progresses*, and at that point the AgentRun controller reads the **live Agent** by name (`prompt`, agent-level `params`, `skillCards`, `execution`). No Agent `generation` is recorded anywhere.

Result: **editing the *Agent* mid-run affects not-yet-created stages.** Example: a 3-stage run on Agent-A; while stage 1 runs, someone edits Agent-A's prompt; stage 2 (also Agent-A) is created later and picks up the edited prompt. The workflow is frozen; the agents behind it are not.

(Note: a *running* stage is already immune in effect — its prompt/params are rendered and baked into the pod at sandbox creation. #169 also tightens the AgentRun to validate/read its Agent exactly once, at creation. This issue is specifically about **later, not-yet-created** stages.)

### Options

1. **Snapshot the Agent definition** per stage (like we do for the workflow) → immune to edits, but larger status payload and a second snapshot to reason about.
2. **Generation-pin**: record each Agent's `metadata.generation` on the stage/AgentRun and detect drift → smaller, but detection is not immunity (need a policy: refuse? warn? proceed?).

Decision (snapshot vs pin, and the drift policy) to be made here, likely with an ADR note extending 0018.

### Related
- #87 (workflow-def snapshot), #169 (controller params/execution/succeeded), ADR 0018.

Start by reading ADR 0018 and tracing the AgentRun controller's creation path, including how later workflow stages resolve their Agent definitions. Compare the snapshot and generation-pin options, then document the chosen approach and drift policy; done means later stages follow that recorded policy rather than silently using changed Agent definitions.

Written by the indexing model from the issue text.