docs: SEO round 2 (Bing) - demote per-page h1 to h2 site-wide, re-trim meta description, bump README/Pages to v0.2.1

Bing Webmaster re-flagged the site after the first SEO pass (32f193a0):
- Every Jekyll-processed doc page (building.md, dev-process.md, internals.md,
  porting-notes.md, dev/agentic-development.md, plus index.md caught last
  time) opens with its own "# Heading" on top of the Cayman theme's masthead
  <h1 class="project-name">, so all of them emitted two h1 tags, not just the
  homepage. Demoted each page's opening heading to h2 -- anchor slugs are
  unaffected (kramdown ids don't depend on heading level).
- index.md's meta description was 161 chars (1 over Bing's 160 limit) and
  still said v0.2.0; re-trimmed to 133 chars and bumped to v0.2.1.
- README.md and docs/index.md's Status lines/comparison table bumped from
  v0.2.0 to v0.2.1 to match the actual current release.

Verified against a live, cache-busted fetch of the deployed Pages site
(the first Bing report was reading a stale CDN edge cache from before the
32f193a0 deploy, per its own Last-Modified header) that the underlying fix
was already live; this pass addresses what was still genuinely wrong.

Claude-Session: https://claude.ai/code/session_01KwZViAFaXua3PKgHHjNgvi

docs/dev/agentic-development.md

@@ -3,7 +3,7 @@ title: Agentic Development
 description: A case study in two AI coding agents porting a Nintendo 64 game to PC; the setup, timeline, handoff workflow, and an honest assessment of what worked.
 ---
 
-# Agentic development: two AI coding agents porting GoldenEye 007
+## Agentic development: two AI coding agents porting GoldenEye 007
 
 *How this port was actually built: a local open-weight model (Qwen 3.8 on a
 single RTX 5090) and a hosted frontier model (Claude / Claude Code) handing