feat: validate input image before creating output dirs

[rasros]

What

Previously run_vector_search called storage.initialize() (which creates the <name>/runs/<timestamp>/nodes/ output tree) before loading the reference image. A missing or corrupt input therefore left an empty project directory behind on every failed invocation.

Now the image is loaded and validated first; storage.initialize() only runs once the input is known-good. On failure, nothing is written to disk.

_load_image is also hardened to distinguish the two failure modes:

• missing path → FileNotFoundError (rendered by main.py as Error: input image not found: <path>)
• exists but not decodable → ValueError: input image could not be read as an image: <path> (...) (rendered as a clean Error: line)

Together with PR #5 this closes the missing-image handling: clean message and no stray directories.

Tests

Added two fast tests (no LLM/workers reached, so unmarked) asserting a missing path raises FileNotFoundError and a corrupt file raises ValueError, and that in both cases storage.project_dir is never created. Full suite: 363 passed.