Update attach snapshot and soften context_values wording

The preamble removal changed the `stage.completed` payload, so the
`attach --json` inline snapshot no longer matched. Drop the stale
`current.preamble` line.

Reword the `context_values` doc row. `stage_context_values` only strips
runtime-only keys; it does not normalize artifact pointers to blob refs
the way `artifact::durable_context_snapshot` does, so calling it a
durable snapshot overstated it. Point readers at `checkpoint.completed`
for the durable projection.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/internal/events.md

@@ -424,7 +424,7 @@ Emitted when a workflow node finishes execution.
 | `failure_signature` | string? | Dedup key for repeated failures |
 | `context_updates` | object? | Context delta written by this stage |
 | `jump_to_node` | string? | Non-edge jump target |
-| `context_values` | object? | Durable context snapshot after the stage; runtime-only keys such as `current.preamble` are omitted |
+| `context_values` | object? | Context snapshot after the stage, minus runtime-only keys such as `current.preamble`. Artifact pointers are not normalized to blob refs — use `checkpoint.completed` for the durable projection |
 | `node_visits` | object? | Node visit counts after the stage |
 | `loop_failure_signatures` | object? | Loop failure signature counts |
 | `restart_failure_signatures` | object? | Restart failure signature counts |

lib/apps/fabro-cli/tests/it/cmd/attach.rs

@@ -1190,7 +1190,6 @@ fn attach_json_errors_without_prompting_for_human_input() {
         "properties": {
           "attempt": 1,
           "context_values": {
-            "current.preamble": "Goal: Wait for approval/n",
             "current_node": "start",
             "graph.goal": "Wait for approval",
             "internal.fidelity": "compact",