pi-goals: discriminator/failure-mode format + visible sign-off judge

Replace done_when with a discriminator + subtle-failure-mode pair as the
heart of each goal. The discriminator is the POSITIVE success observation
that no failure mode could fake, not just failure-avoidance: a run can
dodge every trap and still produce nothing. Carried through planDrafting,
the sign-off judge, README, and the parser doc.

Format migration: flat numbered markdown goals (`1. [/] goal: ...`),
keyword-anchored parsing (indentation cosmetic), goals matched by text,
subtask states [ ]/[/]/[x]/[-] plus ~~strike~~. Evidence empty at
planning, filled at sign-off, multi-line supported.

CompleteGoal now returns the judge's reasoning under a
`--- sign-off judge ---` block (was just "Signed off"), so the verdict is
visible. Plan mode is read-only: edit/write (except goals.md) and
mutating bash are blocked by a tool hook.

17 parser tests, typecheck + biome clean.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

.gitignore

@@ -1,5 +1,6 @@
 node_modules/
 dist/
 *.log
+.pi/
 docs/reviews/raw.jsonl
 docs/reviews/err.txt