Richer probe metadata in -info --json

[swperb]

Purpose: agents make smarter decisions (alpha? grayscale? huge file? rotated?).
Ease: 🟡 Mod — we force RGBA on load and discard source info; capture it before normalizing.

Tasks

• add to -info --json: has_alpha, color_space (rgb/gray), bit_depth, file_size_bytes
• EXIF where present (orientation, make/model, date) — share the EXIF parser with EXIF auto-orient on load #17
• document the new fields in AGENTS.md

[swperb]

Implementation notes — most of this is nearly free:

• img_load() in src/image.c already calls stbi_info(path,&w,&h,&n) before decoding. n is the source channel count → has_alpha = (n==2||n==4), color_space = (n==1||n==2)?"gray":"rgb". Capture n before we force req_comp=4.
• bit_depth: stbi_is_16_bit(path) (8 vs 16).
• file_size_bytes: stat() (already used for output size in main.c).
• These need no full decode — stbi_info + stbi_is_16_bit are header-only/cheap. Consider a dedicated img_probe() returning an ImageInfo struct so -info doesn't decode pixels at all.
• EXIF (orientation/make/model/date) is the only hard part — needs a small EXIF parser; share it with EXIF auto-orient on load #17 (EXIF auto-orient) and land it as a second commit. Do the easy four fields first.