Migrate blog from MkDocs to a Jac web app; rework publishing infra and scheduling workflow

[Jayanaka-98]

Summary

This PR replaces the MkDocs Material site with a Jac web app and reworks the editorial infrastructure around it — publishing, scheduling, and CI. It's the largest structural change to the repo since launch (~6.7k lines across 42 files).

What changed

1. Site rebuilt as a Jac app (no more MkDocs)

The static MkDocs build is replaced by a Jac Cloud app with a React/Vite client.

• Backend (main.jac) — walkers parse posts/*.md and author metadata from the content dir and serve them as JSON. draft: true posts are filtered; posts identify by slug: frontmatter, not filename.
• Routes (pages/) — index, blog/index, blog/posts/[slug], blog/categories/{index,[name]}, blog/authors/[id], blog/archive, blog/search, a shared layout, and a [...notFound] catch-all.
• Components (components/) — PostBody and RunnableJacBlock (interactive Jac code blocks, ported from the old MkDocs/Pyodide setup).
• Styling — new styles/typography.css, styles/prose.css, styles/main.css for readability and prose layout.
• Project config (jac.toml) — serve config, client plugin (Vite/React deps), and the Kubernetes scale plugin for deployment.

The old docs/blog/ content tree is kept as the read-only content source so existing posts keep working.

2. Publish + scheduling workflow rework

Scheduling is now decoupled from merge and driven from a GitHub issue instead of the PR. New maintainer reference: .github/RELEASE_FLOW.md.

• Post URLs are slug-only and permanent (/blog/posts/<slug>): the app serves an in-page "coming soon" state for an unpublished slug and the full article once live — same link throughout. No placeholder page, no date in the URL.
• New post-ci-automation.yml — on lint success, opens/updates a scheduling-tracker issue (and a docs-reference issue), and posts one sticky comment on the PR linking both.
• Editors comment /schedule <ISO8601 UTC> on the tracker issue → scripts/schedule_lib.py writes .schedule.yml on main.
• Reworked auto-publish.yml — hourly cron flips due posts live (removes draft:, rewrites date: on first publish), closes the tracker issue, and comments the live URL.
• Updated lint-new-posts.yml and slash-schedule.yml to match the new flow.

3. Test suite + CI

• New test.yml — runs on every PR (intended as a required check alongside Lint new posts).
• Unit tests for the editorial tooling: tests/test_schedule_lib.py, structural/YAML checks on the workflows in tests/test_workflows.py, shared fixtures in tests/conftest.py.

4. Misc

• New logo/assets, public/ symlink for static serving, updated README.md to describe the Jac app.
• Removed the accidentally-committed .jac/data/server.db* files and added them to .gitignore.

Notes for reviewers

• This is a migration PR — the MkDocs site and the Jac app coexist in the tree during the transition; the content under docs/blog/posts/ is unchanged and remains the source of truth.
• Editorial invariants are preserved: draft: true is still the publish gate, .schedule.yml stays bot-owned via schedule_lib.py, and posts are still keyed by slug:.