docs: rewrite README for first-time readers (jargon-free front page)

> 🤖 Generated by the Agentic Engineer (Cursor cloud instance)

Part of devantler-tech/monorepo#2225 — real sub-issue link for the remaining `ksail` README rewrite
(the epic's prose checklist is not a GitHub sub-issue relationship).

## Evidence

monorepo#2225 verified several portfolio READMEs open with author-memory internals. `ksail` is still
unchecked on that epic. Sibling repos already rewritten: monorepo, platform, actions; world-at-ruin /
dotnet-template / homebrew-tap already meet the bar.

## Audience / problem

A first-time visitor to `devantler-tech/ksail` needs "what is this, is it for me, what do I run first"
in the first screen — not an internals manual.

## Hypothesis

A README whose first two sentences say what KSail is and who it is for, with getting-started commands
copy-pasteable and correct, gets a newcomer to first value faster.

## Success signal (from parent)

- [ ] First two sentences say what it is and who it is for.
- [ ] Front page fits roughly one screen; reference material is linked, not inlined.
- [ ] Every unavoidable technical term is explained on first use, in common words.
- [ ] Getting-started commands are copy-pasteable and currently correct.
- [ ] Internal maintenance and convention detail lives in `AGENTS.md`, not the README.

**Guardrail:** nothing a reader needs may be deleted — move detail to `AGENTS.md` or a linked doc.

## Size

S.

Read the current README and the parent checklist in monorepo#2225 first, then verify the existing getting-started commands in the repository. Keep the first screen focused on what KSail is, who it is for, and how to start; move internal maintenance detail to AGENTS.md without deleting reader-needed information.

Written by the indexing model from the issue text.