docs: introduce the Codex Usage Analyst public identity

[douglasmonsky]

Goal

Make the repository front page a compelling, accurate public introduction to Codex Usage Analyst, while preserving the established Codex Usage Tracker name, traffic, install paths, and technical identifiers during a gradual brand transition.

Scope

• Lead with Codex Usage Analyst — formerly Codex Usage Tracker.
• Explain the agent-first local usage kernel and Evidence Console value proposition.
• Restore the established CI, PyPI, download, Python, and license badges.
• Add an explicit-value hero and current synthetic Evidence Console screenshots.
• Include copy-paste agent installation and meaningful analysis prompts.
• Clearly distinguish shipped behavior from future live-sidecar direction.
• Keep repository, PyPI package, CLI, plugin identifier, MCP namespace, URLs, and install commands unchanged in this phase.
• Add bounded checks for public claims, links, identifiers, and deterministic assets.

Acceptance

• README renders correctly on GitHub and retains old-name discoverability.
• All public claims match current shipped behavior.
• Focused documentation/package checks pass.
• Focused PR merges only after CI succeeds.