fix(core): throw on unimplemented migration sources instead of silently no-oping

[Alexi5000]

Summary

• Replace silent console.log stubs in migrateChatGPT(), migrateClaude(), and migrateGemini() with explicit throw new Error() calls
• Prefix unused inputPath parameters with underscore to satisfy linters
• Callers of migrate() now get a clear error instead of a successful no-op when using an unimplemented source

Motivation

The current stubs in platform/core/src/migrate.ts log a message and return successfully, which means callers have no way to distinguish between a successful migration and one that did nothing. This can mask issues in tooling that depends on migrate().

Test Plan

• Calling migrate({ source: 'chatgpt', inputPath: '...' }) now throws with a descriptive error
• Same for claude and gemini sources
• The default case in the switch still throws for unknown sources

[PR-Reviewer-Ant]

Hi @Alexi5000 - I'm taking a look at the fixes in fix(core): throw on unimplemented migration sources instead of silently no-oping (commit 8e98444c) and will follow up shortly.

This comment is updated in place by pr-reviewer.

[PR-Reviewer-Ant]

Review metadata

• Reviewer: pr-reviewer
• Model: gpt-5.5
• Commit: 8e98444c

The implementation matches the PR description: the three currently unimplemented migration sources now fail explicitly instead of logging and returning success. I did not find a correctness, security, or data-integrity issue in this diff.

Confidence: High [sufficient_diff_evidence, targeted_context_included] - The changed file is small and the diff directly covers the described behavior in platform/core/src/migrate.ts. Each implemented switch branch still dispatches as before, and each unimplemented source now throws a descriptive Error instead of silently returning.