[BUG] Budget regression tests omit required child runner skills argument

Describe the bug

Seven budget pause/reserve regression tests fail on current main before their child agents start. The three test call sites for strix.core.execution._start_child_runner omit its required keyword-only skills argument (introduced by #1553).

To Reproduce

On unchanged main at 09536cf1481731c790995bd8060db19a3b75ff48:

uv sync --frozen --python 3.12
uv run --frozen pytest -q tests/test_budget_pause_policy.py tests/test_e2e_budget_lifecycle.py

Result: 7 failed, 14 passed. Each failure ends with:

TypeError: _start_child_runner() missing 1 required keyword-only argument: 'skills'

The affected calls are the shared start_child test helper in test_budget_pause_policy.py and the two direct child starts in test_e2e_budget_lifecycle.py.

Expected behavior

The fixtures should supply an explicit empty skill selection so these existing tests reach their budget pause/resume and reserve assertions. Production's required argument and skill-specific turn-limit behavior should be preserved.

System Information:

• OS: macOS; the current upstream Linux CI also reports failures in these test sections.
• Strix commit: 09536cf1481731c790995bd8060db19a3b75ff48
• Python: 3.12.14, frozen lockfile
• LLM: none; tests use fake runners/hooks.

Additional context

Found while investigating CI for #1654. The same failure reproduces without that PR. #1577 already addresses a different group of stale telemetry settings in runner tests; this report does not duplicate that fix. The long-running upstream pytest jobs are a separate diagnostic investigation, and correcting these argument mismatches alone is not evidence that the full CI passes.