fix(pi-subagents): terminate children recursively when parent process dies

[tasercake]

Summary

Child Pi subagents now self-terminate when their owning parent Pi session/process dies. This uses an extension-local anonymous lifeline pipe on stdin (fd 0) — zero extra FDs, zero new OS objects, zero external dependencies.

Architecture

• Each parent spawns each child with stdio: [pipe, pipe, pipe] — stdin becomes the lifeline
• Parent holds child.stdin open without writing; the child runtime watches fd 0 for EOF
• When the parent process dies (even abruptly via SIGKILL), the kernel closes all FDs; the child sees EOF on fd 0 and self-terminates via SIGTERM
• Recursive cascade is automatic: every child uses the same mechanism, so parent death → child EOF/termination → grandchild EOF/termination
• Lifelines are closed on session_shutdown (session replacement) but NOT on agent_end (background children survive turns)
• Normal child completion also releases the lifeline
• Timeout semantics unchanged: timeout notifies and does not kill

Changes

File	Change
src/runs/shared/subagent-prompt-runtime.ts	Export PI_SUBAGENT_LIFELINE_FD env constant; add module-level lifeline watcher that listens for EOF on the lifeline fd and self-SIGTERMs
src/extension/index.ts	Import lifeline env; change spawn stdio to [pipe, pipe, pipe]; manage lifelines map; close on completion and session_shutdown
test/unit/minimal-subagents.test.ts	6 new tests covering the full lifeline contract

Validation

• 40 tests pass (1 pre-existing skip unchanged), including all prior tests preserved
• 6 new lifeline tests all green:
  • lifeline env constant export — the runtime exposes the env var
  • prompt runtime sets up EOF watcher — the runtime registers the watcher when env is present
  • session_shutdown terminates children via lifeline — closing lifeline kills real child process
  • agent_end does NOT kill child process — background children survive turns
  • abrupt parent SIGKILL cascades to child termination — killing a real parent process causes its real child to exit promptly
  • recursive cascade — grandparent death kills parent subagent — killing the grandparent causes the parent subagent to die (and by extension any grandchildren)
• Headless liveness test updated to match new stdio pattern and still passes
• All timeout, cohort, widget, and notification tests unaffected