docs: tighten code-reduction claim to defensible '~100x / orders of m…

…agnitude'

Self-correction: the previous commit still hard-coded '1% of the code'
in north-star.mdx, contradicting the explicit recommendation to use
'orders of magnitude' / '~100x' so the claim survives a 'cloc'
challenge.

Also unify the leading message across README, the docs landing page,
and the concept docs to the same crisp framing:

  'Built for AI, not retrofitted' ->
  '~100x / two orders of magnitude less code to read, write, and
   maintain' ->
  'fit in an agent's context window' ->
  'AI as co-maintainer, not autocomplete'

Changes:
- README.md
  - Lead the new Key Feature bullet with 'Built for AI, not
    retrofitted' and use 'roughly two orders of magnitude less code'
  - Restructure the 'Why AI-native?' section with the same
    'Built for AI, not retrofitted' topic sentence and split the
    argument into three paragraphs (assumption -> ~100x -> moat)

- content/docs/index.mdx
  - Replace the standalone '~100x less code' callout with a
    'Built for AI, not retrofitted' callout that mirrors README

- content/docs/concepts/north-star.mdx
  - Replace 'fits in roughly 1% of the code' with 'roughly two
    orders of magnitude less code (~100x fewer lines to write and
    maintain)' in the 'Compact by Construction' tenet

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>