fix: merge synthetic task's input config with parent cache config

[branchseer]

Summary

• When a synthetic task (e.g., vp lint → oxlint) specifies input globs in its cache config, those were silently dropped during the merge with the parent task's config. Now resolves and merges them into the parent's ResolvedInputConfig.
• The parent's includes_auto flag is preserved — the user's explicit choice always takes precedence over the synthetic handler's preference.
• Threads package_dir separately from cwd so glob patterns are resolved relative to the correct base directory.

Test plan

• 8 unit tests covering all merge cases: no input, globs merged, auto ignored, negative globs, cache disabled, etc.
• All existing plan snapshot tests pass
• Cross-platform paths in tests (Unix + Windows)

🤖 Generated with Claude Code

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