refactor: hand-authored pi-flow post + consolidated agents.md

- pi-flow-native-brain is now hand-authored HTML (direct SVGs + styled
  tables; no build.js passthrough needed)
- Card hardcoded in _index_template.html above {{CARDS}}
- Removed posts/pi-flow-native-brain.md (build.js no longer touches it)
- New agents.md: consolidated agent guide for the blog repo
  (blog architecture, build pipeline, adding posts, styling rules,
  writing guide, deploy instructions, skills reference)
- Removed old skills/blog.md (content migrated to agents.md)

skills/blog.md

@@ -1,57 +0,0 @@
-# Skill: Blog Authoring
-
-Write and publish markdown blog posts with YAML frontmatter. This skill teaches an AI agent how to create well-structured blog posts for a static site built from markdown.
-
-## Post Format
-
-Create a markdown file in `posts/<slug>.md` with this frontmatter:
-
-```yaml
----
-title: "Your Post Title"
-slug: your-post-slug
-date: "2026-05-22"
-description: "Full meta description for SEO (150-160 chars ideal)."
-og_description: "Shorter OG/Twitter description."
-og_image: "https://your-domain.com/img/og-cover.jpg"
-excerpt: "Card text shown on the blog index page."
-author: "Author Name"
-author_initials: "AN"
-author_role: "Role, Company"
----
-```
-
-## Writing Guidelines
-
-- **First paragraph** becomes the lead (displayed prominently below the title, separate from the body)
-- **Everything after the first blank line** is the post body
-- Use standard markdown: `## Headings`, `**bold**`, `*italic*`, `[links](url)`, `- lists`, fenced code blocks
-- Images on their own line become `<figure>` elements with captions
-- Use `---` for section breaks
-- Em dashes: `---` renders as &mdash;
-
-## Structure
-
-A good technical blog post follows this pattern:
-
-1. **Lead paragraph** --- what this post is about, in one sentence
-2. **The Problem** --- what pain point or question motivated this work
-3. **The Approach** --- what you built or decided, and why
-4. **Technical Details** --- how it works, with code/diagrams
-5. **What We Learned** --- insights, surprises, trade-offs
-6. **Closing** --- what's next, or an invitation to the reader
-
-## SEO Checklist
-
-- [ ] Title under 60 characters
-- [ ] Description 150-160 characters
-- [ ] og_image set
-- [ ] Meaningful excerpt for index card
-- [ ] Internal links where relevant
-
-## Conventions
-
-- Slugs are kebab-case, matching the filename: `my-post.md` -> slug `my-post`
-- Dates are ISO format: `2026-05-22`
-- Canonical URLs: `https://your-domain.com/blog/<slug>`
-- Don't edit generated HTML --- edit the markdown, then rebuild