Reflection follow-up: rewrite missing substrate file LINEAR-VS-FLEET-TASKS.md

Missing path: ~/shared-knowledge/LINEAR-VS-FLEET-TASKS.md Referenced in: AGENTS.md lines 51,99,118 and HEARTBEAT.md line 237. Presumed contents: Explains the dual-tracking model of Linear issues (milestone-level) vs fleet-tasks (atomic subtask level), anti-patterns for conflating the two, and a worked example (i18n/language preference). AGENTS.md describes it as documenting when to use BOTH systems, how they link via linear_issue_id, and includes a worked i18n example. This is a critical operational document since AGENTS.md mandates reading it for all non-trivial work.