Add P3c-1 child args and JSONL monitor

[lantisprime]

Summary

• add safe child Pi argv construction helpers without spawning or executing child processes
• add stdin/private-temp-file prompt transport planning with redacted argv previews
• add bounded fake JSONL reducer for child session metadata, assistant summaries, tool trajectory, usage/cost/stopReason/model/provider, errors, and truncation flags
• export new helpers from the agents extension entrypoint
• add focused negative tests and P3c-1 test runner

Tests

• agents/test-fixtures/run-p3c-1-tests.sh
• scripts/test-security-scan.mjs
• git diff --check

Coverage

• no prompt/task text in argv
• default argv includes --mode json, --no-session, --tools, and -p
• --approve is not added
• run_subagent/bash/edit/write are rejected as child tools
• unsafe argv tokens and path controls are rejected
• unsupported prompt transports and invalid reducer limits are rejected
• malformed/oversized fake JSONL lines are handled
• stdout, line, summary, tool-args/result, and tool-call truncation flags are covered
• thinking content is excluded from summaries

Scope notes

• no live child Pi execution
• no actual subprocess spawning
• no /agents run implementation
• no run_subagent implementation
• no chain mode
• no command execution