Fix pi-subagents result path env alias handling

[tasercake]

Summary

• fix the result-path guidance regression introduced by 3dbd5fa / PR docs(pi-subagents): clarify result file guidance #19 by telling child agents to pass the resolved absolute path to Pi file tools and reserve $PI_SUBAGENT_RESULT_PATH for shell commands
• add a child-runtime compatibility guard that rewrites only the exact $PI_SUBAGENT_RESULT_PATH and ${PI_SUBAGENT_RESULT_PATH} path tokens for write, edit, and read
• leave embedded variables, arbitrary env vars, unrelated paths, missing-env calls, and bash commands untouched
• replace copied prompt-construction tests with tests of the registered production handlers and document the distinction

Validation

• observed RED: focused regression test failed because no tool_call guard was registered
• npm test in extensions/pi-subagents (35 passed, 1 opt-in real-model integration skipped)
• npm pack --dry-run --json in extensions/pi-subagents
• git diff --check