Centralize CLI color theme into reusable theme object (#230)

* Standardize CLI colors to match the rainbow palette in --help

All chromatic colors used across CLI output now sample from the same
soft, fog-filtered HSL gradient (hue 12°–282°, S 45%, L 62%) that the
"Universal" rainbow header uses, via a shared `theme` export in
src/cli/output.ts. Replaces direct `chalk.red`/`chalk.yellow`/etc.
calls so red errors, yellow warnings, cyan accents, etc. all share the
same tonal range and feel consistent with the help banner.

Neutral modifiers (chalk.gray, chalk.dim, chalk.bold) are kept as-is —
they have no "tone" to standardize. keychain.ts is left on chalk.red
directly to preserve the lib → cli layering boundary.

* Make the "live" green actually feel alive

The session status indicator uses theme.green, which at the rainbow's
pastel S 45% / L 62% read as a washed-out sage rather than a healthy
"alive" green. Bumps theme.green specifically to hsl(135, 65%, 52%)
(#35d45d) — more saturated and slightly darker — while leaving the
other rainbow colors untouched.

Refs #230

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Co-authored-by: Claude <noreply@anthropic.com>