Format command output for terminal workflows

[brdv]

Summary

Apply consistent human-readable formatting across every stck command while preserving automation-safe redirected output.

This is stack 3/3 and is based on #63 (subprocess-progress). Terminal output gets distinct stck, git, and gh labels, compact sync/push step rows, structured stack status, colored success and error markers, and clearer recovery blocks. Redirected output retains the established $ git, error:, and Summary: strings without ANSI sequences.

Changelist

• src/ui.rs: add centralized info, success, operation, diagnostic, and status renderers with exact plain-output fallbacks.
• src/commands.rs: route all user-facing command output through the renderer.
• tests/status.rs: enforce ANSI-free captured output.
• USAGE.md: document terminal progress, subprocess diagnostics, NO_COLOR, and redirected-output guarantees.

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