docs: update Learning Hub for Copilot CLI v1.0.63–v1.0.64 changes (#2107)

- understanding-mcp-servers: add deferTools config option (v1.0.63),
  update /mcp install section to reflect new /mcp registry command (v1.0.64)
- agents-and-subagents: update rubber-duck agent section to reflect
  /subagents configuration and complementary model strategy (v1.0.64)
- creating-effective-skills: add argument-hint frontmatter field (v1.0.64)
- copilot-configuration-basics: add model family aliases (opus/sonnet/haiku/
  gpt/gemini) and /branch alias for /fork (v1.0.64)
- automating-with-hooks: add fix note for postToolUse pipe-separated
  matchers (v1.0.63)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/creating-effective-skills.md

@@ -3,7 +3,7 @@ title: 'Creating Effective Skills'
 description: 'Master the art of writing reusable, shareable skill folders that deliver consistent results across your team.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-05-08
+lastUpdated: 2026-06-23
 estimatedReadingTime: '9 minutes'
 tags:
   - skills
@@ -136,6 +136,24 @@ The `description` field is critical for agent discovery. Write it so that agents
 
 Include trigger keywords and contextual cues that help agents match the skill to user intent.
 
+### Optional Fields
+
+**argument-hint** *(v1.0.64+)*: A short label that appears in the slash-command input placeholder to guide the user on what argument to provide. For example, a `generate-tests` skill might set:
+
+```yaml
+argument-hint: 'Enter function or file to test'
+```
+
+When the user types `/generate-tests` in VS Code Chat, the hint appears as placeholder text in the input box, making the expected input immediately obvious.
+
+```yaml
+---
+name: generate-tests
+description: 'Generate comprehensive unit tests for the selected code, covering happy path, edge cases, and error conditions'
+argument-hint: 'Enter function, class, or file to test'
+---
+```
+
 ## Real Examples from the Repository
 
 The awesome-copilot repository includes skill folders demonstrating production patterns.