[Feature]: Add manifest input for `paperbanana sweep` to replace long CLI flag chains

[eureka0928]

Problem or motivation

paperbanana sweep today takes eight comma-separated axis flags (--vlm-providers, --vlm-models, --image-providers, --image-models, --iterations, --optimize-modes, --auto-modes, --max-variants) plus --input and --caption. A realistic sweep command is a 200+ character line of bash that's miserable to retype, miserable to diff, and miserable to commit to a paper repo alongside the methodology source.

paperbanana batch already solved this with YAML/JSON manifests. Sweep is the obvious symmetric gap — the build_sweep_variants() helper already accepts lists, so all the machinery is in place.

Proposed solution

Add a --manifest / -m option to the sweep command that loads a YAML or JSON file:

# examples/sweep_manifest.yaml
input: examples/sample_inputs/transformer_method.txt
caption: "Overview of encoder-decoder architecture with sparse routing"
pdf_pages: "1-5"            # optional
max_variants: 20            # optional

axes:
  vlm_providers: [gemini, openai]
  vlm_models: [gemini-2.5-flash, gpt-4o]
  image_providers: [google_imagen, openai_imagen]
  image_models: []
  refinement_iterations: [2, 3]
  optimize_inputs: [false, true]
  auto_refine: [false]

Then:

paperbanana sweep --manifest examples/sweep_manifest.yaml

Semantics

• --manifest is mutually exclusive with the eight axis flags (fail early if both are set, mirroring how batch-report errors on --batch-dir + --batch-id).
• input and caption are required keys; all other keys default to empty/None just like the CLI flags.
• Paths inside the manifest resolve relative to the manifest's parent directory (same convention as load_batch_manifest).
• --output-dir, --format, --config, --dry-run, --auto-download-data, --verbose stay as CLI flags (they are invocation concerns, not sweep-plan concerns).

Scope

• New load_sweep_manifest(path) helper in paperbanana/core/sweep.py: ~25 LOC.
• Wire --manifest into the sweep CLI command: ~20 LOC.
• Tests in tests/test_core/test_sweep.py (valid YAML + JSON, missing required keys, wrong types, mutual-exclusion): ~50 LOC.
• examples/sweep_manifest.yaml and one README line.

Estimated functional total: ~50 LOC.

Alternatives considered

• CLI config profiles (shelf of pre-set flag combinations). Rejected because it's a new concept; manifest mirrors existing batch UX.
• Reuse batch manifest shape with a sweep: section. Rejected: batches run different figures, sweeps run variants of one figure. Overloading the shape would confuse both paths.

Area

CLI

Willingness to contribute

• I'd be willing to submit a PR for this feature