docs: rewrite README as a landing page; move reference content into docs/ (#68)

The README was a landing page welded to the pre-Diátaxis reference manual
(863 lines, first code at line 187, a Vercel comparison table before Install).
Rewrite it as a ~200-line landing page: value proposition, a complete
build-checked hello-world Slack bot, install, feature highlights linking into
docs/, adapter maturity, the Diátaxis index, the Vercel relationship in three
sentences, short non-goals, and status.

Relocate every removed reference section into its Diátaxis home: runtime
semantics by concept plus per-adapter status and the testing contract into
docs/reference.md; design goals, the Vercel comparison table, the long-form
non-goals, and the intentional gaps into docs/explanation.md. Dedupe against
existing how-tos rather than paste, and fix two stale how-to claims found on
the way (lost lock lease cancels the handler with ErrPreempted; block_actions
admission rejection is a 503, only slash commands get the busy text).

documentation_test.go: retarget every README phrase assertion to the phrase's
new home (none removed) and add TestREADMEMarkedSnippetsBuild, which compiles
the README's marked Go block against the module.

Author: ThomasK33