Capture real UserPromptSubmit stdin fixture per #664 §8.7

[michael-wojcik]

Problem

pact-plugin/tests/fixtures/userpromptsubmit_stdin_post_bootstrap.json is a synthetic placeholder fixture for the bootstrap_marker_writer.py writer's stdin shape. PR #671 (which closed #664) shipped this synthetic fixture per architect §8.7 with a TODO(#NNN) placeholder comment, deferring real-fixture capture to a follow-up.

The synthetic fixture matches the architect-documented platform schema and exercises the writer's parser correctly, but it cannot verify production-shape parity. The wake_lifecycle_emitter precedent (#612 / #638) is the cautionary tale: synthetic fixtures missed the real-world stdin shape divergence.

Capture procedure (architect §8.7)

1. Cut a separate scratch branch off main (capture-only — does not merge).
2. Add a logging shim to bootstrap_marker_writer.py source that writes stdin JSON to a fixture file before normal handling.
3. Run a fresh-startup PACT session through bootstrap (Steps 1-4 + first real user prompt). The shim captures the actual UserPromptSubmit stdin shape.
4. Save the captured JSON as pact-plugin/tests/fixtures/userpromptsubmit_stdin_post_bootstrap.json, overwriting the synthetic placeholder.
5. Remove the shim from the source.
6. Open a small PR replacing the synthetic fixture with the captured one. Substitute the TODO(#NNN) placeholder in the fixture's _comment field with this issue's number.

Why this matters

• Catches any platform-level stdin-shape divergence the synthetic schema misses.
• Makes the fixture-load test (test_captured_fixture_exists + test_main_runs_cleanly_against_fixture) genuinely verify production parity rather than schema-self-consistency.
• Honors the audit-anchor pattern documented in wake_lifecycle_emitter: Arm directive never emits — _extract_task_id misses production tool_response shape #612 and fix: extend _extract_task_id to probe nested tool_response.task.id (#620) #638.

Acceptance

• Captured fixture replaces the synthetic placeholder.
• TODO(#NNN) placeholder in fixture comment substituted with this issue number.
• Existing fixture-load tests continue to pass against the captured shape.
• Follow-up PR notes the capture procedure used so future re-captures can repeat it.

Out of scope: re-capturing on every plugin version bump (only re-capture when suite failure suggests stdin-shape drift, same as wake_lifecycle_emitter precedent).