Fix goal progress verification

[Rohith31-WD]

Summary

• Reject client-supplied progress updates for GitHub-synced goals (commits and prs)
• Keep manual progress updates available for non-GitHub goals
• Add regression tests for synced-goal PATCH behavior

Root cause

The PATCH endpoint validated that current was numeric and within target bounds, but it trusted the client as the source of truth for goals whose progress should come from GitHub activity.

Validation

• npm exec -- tsc --noEmit
• npm exec -- vitest run test/goals-patch-progress.test.ts could not start in this sandbox because the local runner's esbuild subprocess was blocked with spawn EPERM.

Fixes #1753

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GSSoC Label Checklist 🏷️

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