M146: Rework pre-built-schema blog (title, narrative, formatting)

[a6b8]

Closes #136

Follow-up to #134 / PR #135 — improves the published post /blog/2026-06-what-a-prebuilt-schema-saves/.

What changed

• Headline → The Amnesiac Agent: What a Pre-Built Schema Saves in Tokens, Dollars, and CO₂ (uses the post's own Memento / in-context-learning hook).
• Formatting → removed the redundant leading metadata blockquote (BlogPostLayout already renders author/date/tags), opens on the hook, adds a scannable "five numbers" summary table.
• Substance → tighter prose with bold lead-ins; added the full 8-model price table (Memo 146 figure 7, previously unused).

Invariant

All hard numbers kept verbatim from the deterministic Memo 146/143 figure pipeline — cross-checked against generated-figures.md (counts, cost bands, 7-call break-even, 6–47× factor, CO₂ range, model prices). All source citations preserved.

Verification

• astro build green — 177 pages, no errors.
• No German version exists; no test/playwright spec references the old title or slug.