Make fsharp-diagnostics skill a directive, not a suggestion (#19520)

Change wording from 'use this to get...' to 'Always invoke after
editing .fs files. Prefer this over dotnet build for iterative changes.'

The agent treats suggestions as optional but directives as requirements.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

.github/skills/fsharp-diagnostics/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: fsharp-diagnostics
-description: "After modifying any F# file, use this to get quick parse errors and typecheck warnings+errors. Also finds symbol references and inferred type hints."
+description: "Always invoke after editing .fs files. Provides fast parse/typecheck feedback without a full dotnet build. Prefer this over dotnet build for iterative changes. Also finds symbol references and inferred type hints."
 ---
 
 # F# Diagnostics