docs: fix copilot-instructions relative path and add Dependabot to changelog

- Fix broken relative link in .github/copilot-instructions.md
  (../../AGENTS.md -> ../AGENTS.md)
- Add Dependabot auto-merge entry to CHANGELOG Infrastructure section
- Note weekly schedule on security scanning entry

Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>

Author: SebTardif

.github/copilot-instructions.md

@@ -1,3 +1,3 @@
 # Copilot
 
-See [AGENTS.md](../../AGENTS.md) for project conventions, dev commands, and coding standards.
+See [AGENTS.md](../AGENTS.md) for project conventions, dev commands, and coding standards.

CHANGELOG.md

@@ -31,5 +31,6 @@
 ### Infrastructure
 
 - Automated test harness with unit tests and VS Code extension integration tests
-- CI on self-hosted runner with npm audit security scanning
+- CI on self-hosted runner with npm audit security scanning (weekly schedule)
+- Dependabot auto-merge for minor and patch dependency updates
 - Extension packaging with `@vscode/vsce`