Clarify Relay product story on GitHub

[miter37]

What changed

• Reframed the README around Relay's current product model: reusable Tasks, multi-step Projects, Project Runs, Artifacts, optional review gates, Routines, and bounded Orchestrator assistance.
• Added a concise end-to-end workflow and a Core concepts table so the repository landing page explains how the pieces fit together.
• Documented Project Run evidence surfaces (Pipeline, Artifacts, Timeline, Inspector, and Orchestrator), human/Orchestrator review behavior, review feedback and reruns, and the CLI paths for configuring and resolving reviews.
• Updated the package description in pyproject.toml and synchronized the GitHub repository description and discovery topics.
• Recorded the documentation outcome in log.md.

Why

The previous landing page described Task Runs and worker CLIs well, but it did not make Relay's central value proposition visible: turning AI CLI work into durable, inspectable workflows with an explicit publication boundary.

Validation

• README local anchors: 15 checked
• git diff --check
• py -m relay --version
• py -m relay project review-config --help
• py -m relay review rerun --help

The existing cross-platform CI and full test matrix already passed on the preceding application commit. .claude/settings.local.json remains intentionally untracked.