Add workflow_dispatch trigger to CI workflow

[Copilot]

This PR adds the workflow_dispatch trigger to the CI workflow, enabling manual execution of the CI pipeline from the GitHub Actions UI.

Changes Made

• Added workflow_dispatch: to the on: section of .github/workflows/ci.yml
• Preserves all existing triggers (push and pull_request events)
• No changes to job definitions or build/test processes

Benefits

• Allows developers and maintainers to manually trigger CI runs for testing purposes
• Useful for debugging workflow issues or running tests on-demand
• Provides more flexibility in CI management without affecting automated triggers

The change is minimal and backward-compatible - all existing functionality remains unchanged while adding the new manual trigger capability.

Fixes #3.

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