feat(ui): add configurable minimal UI

[mochow13]

Problem

The current terminal UI can be noisy: users may not need to continuously see shell commands, tool details, generated code, or verbose command output while an agent works. They primarily need clear progress and important outcomes.

Proposal

Explore and implement a first solid version of a minimal UI mode that displays only important details, such as:

• The agent’s current or next high-level action
• Permission and confirmation requests
• Errors and failures
• Meaningful file-change and test outcomes
• A concise final summary

Hide or collapse lower-level details by default in this mode, including raw shell commands, tool payloads, generated code, and routine command output.

The mode should be possible to turn on or off through a command.

Open questions

• What should be considered important enough to always display?
• Should minimal mode be the default, opt-in, or remembered per user/session?
• Which command and UX best fit the REPL?
• How can users reveal hidden detail on demand without losing transparency?
• Should this grow into multiple verbosity levels after the initial version?

Acceptance criteria

• A minimal UI mode can be enabled and disabled with a command.
• When enabled, normal agent activity is represented by brief, understandable status updates.
• Important permission prompts, errors, and final outcomes remain visible.
• The initial behavior and command are documented.
• Add focused tests for the selected behavior.

Inspired by feedback from joe-dakroub on Hacker News.