Add reference image guidance for diagram planning

[simplaj]

Summary

Closes #223.

Adds and hardens reference/sketch image guidance for methodology diagram generation. The CLI validates repeatable --image inputs before pipeline start, the Planner attaches user-provided images after retrieved exemplars and labels their attached-image positions explicitly, and the Visualizer gets a lightweight note when a plan was sketch-guided. The Critic continues to judge only the generated output against the source text/caption.

Changes

• Validate --image paths with PIL and normalize them to absolute paths before constructing GenerationInput.
• Preserve existing repeatable --image behavior while improving missing-file/non-raster error messages.
• Label user-provided images in the Planner prompt with exact attached image numbers, offset after successfully loaded exemplar images.
• Record input_image_count in progress and persist input_images in planning metadata.
• Expand planner and CLI tests for image ordering, path validation, missing/non-image inputs, and --continue conflicts.

Verification

• .venv/bin/python -m pytest tests/test_pipeline/test_types.py tests/test_agents/test_planner.py tests/test_cli.py -k "image or input_images" -q -> 22 passed
• .venv/bin/python -m ruff check paperbanana/agents/planner.py paperbanana/agents/visualizer.py paperbanana/cli.py paperbanana/core/pipeline.py paperbanana/core/types.py tests/test_agents/test_planner.py tests/test_cli.py tests/test_pipeline/test_types.py -> passed
• git diff --check origin/main...HEAD -> clean
• git merge-tree $(git merge-base HEAD origin/main) HEAD origin/main -> clean
• GitHub compare: ahead_by=1, behind_by=0

[simplaj]

It appears this issue has already been fixed in other PRs, so I'll close this one for now. Thanks for your review!