Add terminal-aware output foundation

[brdv]

Summary

Introduce a dependency-free terminal-aware rendering foundation for stck.

This is stack 1/3 and targets main. Its child is #63. It detects stdout and stderr terminal capabilities independently, respects NO_COLOR and TERM=dumb, and keeps redirected output plain and grep-friendly.

Changelist

• src/ui.rs: add terminal policy and stable error rendering.
• src/cli.rs: route top-level preflight errors through the renderer.
• tests/status.rs: verify captured errors remain ANSI-free.

Checklist

• Changes are minimal and focused for this milestone
• No breaking CLI changes unless explicitly intended
• Errors are user-facing, actionable, and prefixed with error:
• Added/updated tests for behavior changes
• Updated docs/plan if behavior or scope changed