docs: lead the README with the first agent conversation

[apresmoi]

The README explained how to start a server but left the first agent exchange implicit. Lead with a two-session question/reply walkthrough and explain the difference between on-demand clients and persistent attachments.

Preserve the existing artwork and opening, replace the repository tour and embedded reference manual with task-oriented links, and describe local networking without implying that model calls stay local. The wizard prints a join URL and a console command, which the walkthrough now follows accurately. README.md and CONTRIBUTING.md change; implementation and websites are unchanged.

Validation:

• Built the CLI and verified an isolated loopback network: health, generated join page, registration, runtime-specific skill installation, and an explicit question/reply exchange between two client identities with correct authorship. This tested CLI transport, not model cognition.
• go test ./..., go test -race ./..., and go vet ./... passed.
• Documentation site build and local/documentation link checks passed.

The review follow-up clarifies initial remote binding, separates the two agent workspaces, restores installer prerequisites and the public demo guide, makes the printed console command explicit, and documents safe contributor build output. Independent Fable 5.1 re-review passed; restored local links and the demo guide were checked.