test(pi): scope mapping assertions to the table, not whole file

The pi tokens (subagent, pi-subagents, Task, TODO.md) also appear in the
surrounding prose, so matching the whole file passed even with the mapping
table deleted — the exact regression this test exists to catch. Filter to
table rows (lines starting with '|') so the assertion fails when the table
is gone and passes on dev.

Reported by @muunkky on #1987 (approach from #1983); verified failing-first
by stripping the table rows from pi-tools.md.