Add CIRCLE_ORG fallback for org commands

[ahfoysal]

Adds CIRCLE_ORG as a default for org-scoped commands when --org is omitted.

Tests:

• go test ./acceptance -run '^TestContextList' -count=1
• go test ./internal/cmdutil -count=1
• go test ./internal/cmd/root -count=1

[ahfoysal]

Merged latest main into this branch and resolved the conflict in internal/cmdutil/orgid.go by keeping both the new upstream InferOrgID helper and the CIRCLE_ORG fallback helper from this PR.

Validation:

• go test ./internal/cmdutil ./internal/cmd/context
• go test ./acceptance -run 'TestContextList_OrgFromEnv|TestContextList'