fix(#1359): migrate workflows off deprecated TaskOutput to Read(outputFile) (#1362)

* fix(#1359): migrate workflows off deprecated TaskOutput to Read(outputFile)

The map-codebase and docs-update workflows collected background sub-agent
results with the deprecated Claude Code `TaskOutput` tool using `block: true`,
which has a confirmed main-session hang after the agent completes
(anthropics/claude-code#20236).

Migrate the collection steps to the upstream-recommended pattern: keep
`run_in_background=true` on the Agent spawn, then `Read` each agent's
`outputFile` (from the `async_launched` result) once it reports completion.
Completion-marker contracts and on-disk verification are unchanged, and the
non-Claude runtime fallbacks (sequential_mapping / sequential_generation) are
preserved byte-for-byte. docs-update's timeout note no longer references the
unwired `workflow.subagent_timeout` key (it kept a literal before).

Regression coverage folded into tests/subagent-timeout.test.cjs (the owning
module for background-subagent collection). Workflow size baseline regenerated
for the justified prose growth.

Refs #1355 (same latent hang surface).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#1359): add changeset for TaskOutput migration

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/workflow-size-baseline.json

@@ -21,7 +21,7 @@
   "discuss-phase-power.md": 11273,
   "discuss-phase.md": 31965,
   "do.md": 10068,
-  "docs-update.md": 54770,
+  "docs-update.md": 55662,
   "edit-phase.md": 12883,
   "eval-review.md": 9923,
   "execute-phase.md": 93157,
@@ -40,7 +40,7 @@
   "list-phase-assumptions.md": 4305,
   "list-workspaces.md": 5655,
   "manager.md": 25937,
-  "map-codebase.md": 20360,
+  "map-codebase.md": 20789,
   "milestone-summary.md": 11774,
   "mvp-phase.md": 13582,
   "new-milestone.md": 32422,