docs: improve README onboarding and usage

[egdev6]

Problem

The current README documents the package but does not provide enough visual hierarchy or progressive onboarding for new users to understand installation, the plan/apply workflow, safety boundaries, and verification quickly.

Proposed solution

Redesign the README with a clearer visual hero, navigation, concise quick start, copy-paste plan/apply examples, capability and safety tables, release guidance, and a repository map. Keep every command and claim aligned with the actual Pi package layout and implementation.

Acceptance criteria

• Installation uses the immutable v1.0.0 Git package reference.
• Quick start explains the conversational Pi workflow before low-level executor commands.
• Plan and apply examples use Pi's actual project-local Git package path.
• Capabilities and safety guarantees match the skill, schema, and implementation.
• Release behavior describes independent tag and GitHub Release checks.
• Local links and document anchors resolve.
• Existing tests continue to pass.

Scope and non-goals

This is a documentation-only change to README.md. It does not modify package behavior, workflows, tests, release artifacts, or npm publication settings.