[cross-cutting] [P2] No environment variables documented in any `--help`

Severity: P2
Command: cross-cutting

Category: Discoverability — env vars that affect behavior must be listed in --help.

Repro: Greps across all 13 subcommand --help outputs find zero references to NO_COLOR, CI, WINML_*, HF_HUB_OFFLINE, HF_HOME, HF_TOKEN, TRANSFORMERS_CACHE. (Word matches like "HuggingFace" or "environment" are unrelated.)

Actual: Users must read source to discover that:

• NO_COLOR=1 and CI=true suppress color (works via Rich auto-detection).
• HF_HUB_OFFLINE=1 is honored (verified: winml inspect -m microsoft/resnet-50 succeeds against cache).
• HF_HOME / TRANSFORMERS_CACHE redirect the model cache (used transitively by transformers).
• No WINML_* env vars are documented anywhere.

Expected: Add a single Environment Variables epilog to winml --help listing each variable, what it controls, and one example. Same epilog should appear in any subcommand whose behavior depends on the var (e.g. eval and inspect should mention HF_HUB_OFFLINE).

Why it matters: Env vars are invisible by definition. Without documentation, CI authors guess and copy from blog posts; offline users don't know they can run without the network; debug users don't know how to silence color.