Add P3c-2 built-in child runner

[lantisprime]

Summary

• add command-only built-in child runner for /agents run scout|planner|reviewer <task>
• use P3c-1 child argv and JSONL reducer helpers for child invocation and compact result rendering
• enforce built-ins only; registered user/project agents remain blocked for P3c-3
• add timeout, stdout, stderr, JSON line, and result caps with SIGTERM and SIGKILL fallback
• keep prompt/task out of argv; include --no-approve and disable resource discovery by default
• update diagnostics and P3 slice docs for P3c-2

Tests

• agents/test-fixtures/run-p3c-2-tests.sh
• scripts/test-security-scan.mjs
• git diff --check

Coverage

• successful fake child run through runner
• extension command e2e via injected fake runner
• non-built-in agent rejected before spawn
• nonzero exit formatting
• timeout kill
• output-limit kill
• spawn error
• invalid runner limits
• prompt/task privacy in argv
• no --approve; --no-approve present

Scope notes

• built-ins only: scout, planner, reviewer
• no registered user/project spec execution
• no ephemeral agents
• no run_subagent
• no chain mode
• no parallel execution or broader workflow behavior