Add regression tests for SegmentScopeMode prompt injection

[HenryLach]

Problem

The SegmentScopeMode prompt injection (FULL_TASK vs SEGMENT_SCOPED) was added to fix workers self-scoping to one step, but there are no focused regression tests verifying the prompt content for each mode.

Tests Needed

1. FULL_TASK prompt must:

   • Include SegmentScopeMode: FULL_TASK
   • NOT include Active segment ID
   • NOT include segment-scoped checkbox block ("Your checkboxes for this step:")

2. SEGMENT_SCOPED prompt must:

   • Include SegmentScopeMode: SEGMENT_SCOPED
   • Include Active segment ID
   • Include segment-scoped checkbox block
   • Include "Other segments in this step (NOT yours)"

3. Polyrepo single-segment regression:

   • Worker proceeds beyond Step 0 (does not exit after one step)

4. Legacy/partial marker case:

   • Task with markers on some steps but not others — fallback behavior does not silently one-step scope

Files

• extensions/tests/segment-scoped-lane-runner.test.ts — add prompt content assertions
• extensions/tests/lane-runner-v2.test.ts — update existing segment contracts

References

• Sage review of TP-501 fix
• Polyrepo test failures: workers exiting after Step 0 despite FULL_TASK mode

[HenryLach]

Triage update (2026-05-10): still relevant. Folded into TP-196 — multi-segment engine hardening bundle. TP-196 combines this issue with #462, #502, #503, #508 into a single coherent hardening pass because all four touch overlapping files (lane-runner.ts, execution.ts, resume.ts, discovery.ts) and share the segment-engine mental model. Will auto-close when the TP-196 PR merges.

[HenryLach]

Closed by PR #578 (v0.30.1) — bundled into TP-196 multi-segment engine hardening.