FOLLOW-UP: Thread io.Writer through ParseFile for testable warning output (from PR #173)

[adnaan]

Context

This follow-up task was identified during the review of PR #173.

Source PR: #173
PR Title: refactor: extract shared slug package; add ParseFile warning integration test
Suggested by: @claude[bot] (across 3 review rounds)

Task Description

The TestParseFileDuplicateAnchorWarning test captures warnings by swapping the global os.Stderr with a pipe. This works but is fragile — it's not parallel-safe and mutates global state.

A cleaner approach is to thread an io.Writer parameter through ParseFile (or preprocessAutoTasks) so the test can pass a bytes.Buffer directly. This would:

• Make the warning output testable without global state mutation
• Allow parallel test execution
• Follow Go best practices for dependency injection

Original Comment

A safer pattern is to thread an io.Writer through ParseFile (or preprocessAutoTasks), so the test can pass a bytes.Buffer directly. If the API surface is not worth changing, the test should at minimum add a comment explaining why it must not run in parallel with other tests that write to stderr.

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This issue was automatically created by prmonitor from PR review comments.