fix(docs): restore README.adoc as canonical entry point, remove README.md (#286)

## Summary
On 2026-06-12, a "README dedup" pass kept `README.md` as canonical and
demoted `readme.adoc` to a one-line pointer — backwards from this
estate's own documented policy (AsciiDoc is canonical; no duplicate
`.md` when `.adoc` exists). The dedup's own heuristic for "richest file"
was just file size, which picked exactly the wrong one — a deliberately
thin adoc entry point that correctly delegates to
`docs/echo-types/MAP.adoc` is good documentation design, not a sign of
being less important.

GitHub's README-picking logic favors exact-case `README.md` over
lowercase `readme.adoc`, so **GitHub has been rendering the wrong file
as this repo's main page ever since.**

## Fix
- Converted the full content (previously in `README.md`) to proper
AsciiDoc syntax, now at `README.adoc` (correct casing)
- Removed both `README.md` and the old `readme.adoc` pointer stub
- No content lost — full markdown history remains in git; this is a
syntax conversion, not a content rewrite

## Please check before merging
I don't have `asciidoctor` available locally to render-test this, so I
verified structurally (balanced block delimiters, line-count parity with
the original) but haven't seen it actually rendered. Given how much this
page matters, please look at the rendered PR diff / GitHub's preview
before merging — especially the mermaid diagram, the two ASCII-art
boxes, and the table under "Foundation Contract."

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: hyperpolymath