docs: rewrite README around current install and integration surface

[davidahmann]

Problem

The top-level README was mixing an older proposal narrative with the current shipped Gait surface. That made the install story, integration contract, and framework claims harder to follow than they should be.

Changes

• rewrote the README hero around execution-boundary enforcement, signed proof, and deterministic regressions
• split the funnel into three truthful entry paths: first proof, repo policy onboarding, and runtime integration
• aligned install paths to current Homebrew, go install, and release-installer options
• tightened framework messaging to the current shipped lanes: OpenAI Agents wrapper lane and LangChain middleware with optional callback correlation
• kept MCP trust, observability positioning, regress/CI, and compliance sections aligned with the current CLI and docs contracts

Validation

• gait doctor --json
• make test-docs-consistency
• make test-docs-storyline
• make prepush-full