docs: rewrite the README opening and jargon walls for a first-time reader #2686

🤖 Generated by the Daily AI Assistant

Part of the portfolio-wide README pass — devantler-tech/monorepo#2225.

Problem

The README is accurate but written from the author's memory rather than for a first-time reader:

• The opening says what the platform is, but the only audience signal is "Feel free to look around".
  A stranger cannot tell whether this is a product to adopt or a personal system to learn from.
• The OpenBao prerequisite is a single six-line bullet stacking "SOPS-decrypted seeds",
  "ExternalSecrets", "Dex and oauth2-proxy", and "Flux postBuild substitution" with no plain-word
  glosses.
• The local-cluster paragraph under Usage is a twelve-line wall that inlines three overlay file paths
  mid-sentence.
• The templating section restates mechanics already documented in docs/TEMPLATING.md, and opens
  with a maintainer aside ("If you know of a different way … please let me know") inside an
  [!IMPORTANT] callout.

Acceptance criteria

• Opening states what it is and who it is for, and sets honest expectations about reuse.
• Prerequisites explain each term in common words; no multi-line jargon stack.
• The Usage wall becomes scannable prose plus a list of the overlay files.
• The templating section keeps both diagrams but defers file-level mechanics to
  docs/TEMPLATING.md; the maintainer aside moves out of the README.
• No information a reader needs is lost — component inventory, docs index, and cost table stay.

Size: S. Docs-only; no manifest changes, so no cluster validation applies.