feat(pi-subagents): add global background default

[wolfgangmeyers]

Summary

Adds the documented defaultRunInBackground global setting in ~/.pi/agent/subagents.json.
When set to true, it makes root subagent calls that omit run_in_background launch in the background.

Explicit run_in_background: false still launches in the foreground.
Explicit agent-frontmatter behavior continues to win over the global fallback.
Nested child calls remain foreground by default, and forked root sessions still receive the global default.
The service adapter is unchanged.

Validation

• mise exec pnpm@11.5.2 -- pnpm --filter @gotgenes/pi-subagents run check
• mise exec pnpm@11.5.2 -- pnpm --filter @gotgenes/pi-subagents test — 64 files, 1007 tests passed
• mise exec pnpm@11.5.2 -- pnpm --filter @gotgenes/pi-subagents run lint
• git diff --check

Numeric coverage is not available in this package because it has no coverage script or Vitest coverage provider, and @vitest/coverage-v8 is not installed.
The focused behavioral tests cover the global true/false defaults, explicit foreground override, agent-frontmatter precedence, nested-child non-regression, and forked-root behavior.