Add subprocess progress and output framing

[brdv]

Summary

Add TTY-only progress indicators and isolate native subprocess output from stck output.

This is stack 2/3, based on #62 (output-foundation), with #64 as its child. Long Git and GitHub phases now show transient progress in terminals, successful subprocess chatter is suppressed, and failed git/gh streams are preserved in labeled diagnostic blocks.

Changelist

• src/ui.rs: add the dependency-free spinner, tool command labels, and TTY policy.
• src/commands.rs: wrap preflight, discovery, rebase, push, and PR operations with progress.
• src/gitops.rs and src/github.rs: capture mutating subprocess output and retain full failure diagnostics.
• src/util.rs: format captured stdout/stderr into labeled blocks.
• tests/: cover suppressed success chatter, framed failures, real-Git conflict recovery, and clean non-TTY output.

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