[thomaschristory]

Fixes #192.

Before this change, NSC_SCHEMA was used only to set profile.schema_url, and only when the value was a URL. As a result:

• a local path was silently ignored;
• an unreachable URL quietly fell back to the cached or bundled schema.

The new resolve_schema_override(overrides, env) in nsc/cli/runtime.py returns --schema if set, otherwise NSC_SCHEMA. An empty value counts as unset. build_runtime_context passes the result to resolve_command_model as the schema override. NSC_SCHEMA now behaves exactly like --schema: it loads the path or URL directly, skips the TTL, cache and bundled fallbacks, and a load failure exits 3 with Error: cannot load schema from <source>: <reason> (#189).

$ NSC_SCHEMA=./missing.json nsc dcim sites list
Error: cannot load schema from ./missing.json: not found     # exit 3
$ NSC_SCHEMA=http://127.0.0.1:1/s.json nsc ls x
Error: cannot load schema from http://127.0.0.1:1/s.json: request failed (...)   # exit 3

Tests: the new tests/cli/test_schema_env_override.py covers:

• a local path being used, checked via a schema-only dcim widgets resource;
• a missing file exiting 3;
• an unreachable URL exiting 3 with no bundled fallback;
• --schema winning over NSC_SCHEMA;
• an empty NSC_SCHEMA counting as unset.

Docs: docs/architecture/schema-loading.md, docs/architecture/overview.md, CHANGELOG.

https://claude.ai/code/session_01RD8NgCVtp3yYFd6NSZpbUb