Drop --no-color flag in favor of NO_COLOR env var

[jclusso]

The CLI already auto-detects TTY (suppressing colors when piped) and honors the cross-tool NO_COLOR standard. The dedicated --no-color flag was redundant — NO_COLOR=1 emailable … covers the same use case.

Removes the flag, its ui.SetNoColor plumbing, related tests, and the dedicated "Color output" README section. NO_COLOR stays mentioned in the env vars table.

[Copilot]

Pull request overview

This PR removes the redundant --no-color CLI flag and its plumbing, relying exclusively on the cross-tool NO_COLOR environment variable (plus existing TTY auto-detection) to suppress ANSI styling.

Changes:

• Removed the --no-color persistent flag from the root command and deleted the cmd→ui plumbing that enforced “no color” process-wide.
• Simplified internal/ui.IsTTY by removing the extra forced-disable state, leaving NO_COLOR as the only override.
• Updated documentation and tests to remove --no-color references and coverage, while keeping NO_COLOR documented.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Removes the dedicated “Color output” section and updates the env var table to describe NO_COLOR without referencing the deleted flag.
internal/ui/spinner.go	Deletes SetNoColor / forced no-color state; IsTTY now only honors NO_COLOR and terminal detection.
internal/ui/spinner_test.go	Removes tests specific to SetNoColor / forced flag-driven behavior.
internal/output/human.go	Updates comments to reflect NO_COLOR as the only styling suppressor via ui.IsTTY.
cmd/verify_test.go	Removes tests and imports related to --no-color.
cmd/testutil_test.go	Removes test environment resets for the deleted noColor flag state.
cmd/root.go	Drops the --no-color flag and the PersistentPreRunE propagation to ui.SetNoColor.
cmd/root_test.go	Updates help-surface tests to stop expecting --no-color; removes propagation test.
cmd/context.go	Removes NoColor from cmdCtx and updates related comments.

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