🔒 [security fix] Mask sensitive tokens in GitHub Actions logs

- Added `core.setSecret(token)` to mask the primary GitHub token.
- Added `core.setSecret(githubMcpToken)` to mask the GitHub MCP token.
- Updated `__fixtures__/core.ts` to include the `setSecret` mock.
- Updated `__tests__/main.test.ts` to verify `setSecret` is called for the tokens.

__fixtures__/core.ts

@@ -9,3 +9,4 @@ export const getBooleanInput = vi.fn<typeof core.getBooleanInput>()
 export const setOutput = vi.fn<typeof core.setOutput>()
 export const setFailed = vi.fn<typeof core.setFailed>()
 export const warning = vi.fn<typeof core.warning>()
+export const setSecret = vi.fn<typeof core.setSecret>()