Honor NO_COLOR in the terminal log handler (closes #402)

The TTY log handler always wrote ANSI color, unlike internal/ui, which
disables color when NO_COLOR is set. Add a color flag to TTYHandler,
set from NO_COLOR with the same test internal/ui uses, and route every
color code through a paint helper that returns plain text when color is
off. The README's color sentence now covers log lines too.

Model: deepseek-v4-pro

README.md

@@ -836,6 +836,8 @@ Work planned after 1.0. Loosely ordered by priority.
 Every command's user-facing output is governed by `internal/ui`, in one
 of two ways. Color is enabled when the stream written to is a TTY and
 the `NO_COLOR` environment variable is unset (https://no-color.org/).
+The same rule governs the colorized log lines `internal/log` writes to a
+terminal.
 
 * **Status, progress, warnings, and errors** go through the `internal/ui`
   message methods below: marker-prefixed, colored on a TTY, and — except