[Phase 15] Expand docs, onboarding, and contributor workflow

[cosentinode]

Goal

Make the project easy to resume across multiple sessions and contributors.

Scope

• Expand README with product goals, install instructions, and screenshots/gifs when available.
• Document keyboard and mouse controls.
• Document prompt examples.
• Document guardrail philosophy.
• Document local development workflow.
• Keep docs/mvp-phases.md aligned with GitHub issues.

Non-negotiables

• Code simplicity is a must: docs should encourage small, focused changes.
• Security is a must: docs must explain prompt safety, secrets handling, and destructive operation policy.
• Scalability is a must: docs should explain where new features belong in the architecture.

Acceptance Criteria

• A new contributor can understand what to work on next from README and docs.
• Phase docs and GitHub issues do not contradict each other.
• Common workflows are documented with examples.