chore: cut over docs to fumadocs (phase 7)

[pratyush618]

Phase 7 of the docs migration. Final PR before v0.12 ships. Replaces the Zensical site with the Fumadocs site, decommissions Zensical entirely, and updates every doc-build reference across the repo.

What this PR does

Cutover

• docs-next/ → docs/ — git-mv'd in place so the new Fumadocs site lives where the old Zensical site lived. History preserved on every file (git log --follow works through the rename).
• zensical.toml deleted — config file for the old generator, gone.
• Old docs/ deleted — 103 markdown files (the Zensical content). Everything was already ported to MDX in phases 2–4.

Workflows

• .github/workflows/docs.yml rewritten — replaces the Zensical build (uv run zensical build → actions/upload-pages-artifact) with a pnpm + Next.js flow that runs types:check, lint, and build on every PR (path-filtered to docs/**), and on push-to-master uploads docs/out/ and deploys via actions/deploy-pages@v5. Same Pages source ("GitHub Actions"), same custom domain (docs.byteveda.org), no DNS changes needed.
• .github/workflows/docs-next.yml deleted — temporary CI was scoped to PRs only; folded into the new docs.yml so we have a single source of truth.

Other repo updates

• pyproject.toml — removed the docs = ["zensical"] extra; fixed Changelog URL (was pointing at docs/changelog.md, would 404 after the delete; now points at https://docs.byteveda.org/taskito/docs/more/changelog).
• README.md — replaced taskito.grigori.in (stale personal domain) with docs.byteveda.org/taskito; updated in-text doc paths to the new IA (/guide/execution/prefork/ → /docs/guides/advanced-execution/prefork); cleaned up "crossbeam" → "tokio mpsc" reference (the migration to tokio happened way back; the README hadn't caught up).
• CLAUDE.md — refreshed test counts (77→89 Rust, 469→496 Python, 46→49 test files), added a docs build block (pnpm --dir docs ...).
• CONTRIBUTING.md — replaced the pip install ".[docs]" + zensical serve block with the Fumadocs equivalent (pnpm --dir docs install + dev).
• .claude/memory/docs.md — full rewrite for Fumadocs conventions (stack, IA, MDX patterns, custom components, gotchas).

What does NOT change

• DNS / CNAME — docs.byteveda.org already CNAMEs to GitHub Pages. The new workflow uses the same publish mechanism (upload-pages-artifact + deploy-pages), so the artifact swaps in place.
• basePath: '/taskito' — already lined up with the GH Pages path prefix; production builds emit URLs that match the existing CNAME mapping.
• GitHub Pages settings — already set to "GitHub Actions" as source. No changes needed in repo settings.

Known SEO/UX cost (intentional)

URL paths change from the Zensical layout to the Fumadocs IA. Examples:

• /guide/execution/prefork/ → /docs/guides/advanced-execution/prefork
• /api/queue/ → /docs/api-reference/queue
• /changelog/ → /docs/more/changelog

External inbound links (PyPI page, blogs, Stack Overflow answers) will 404 until they're updated. Server-side redirects aren't possible on static GitHub Pages. This was flagged as a non-goal in the original migration plan.

Test plan

• pnpm --dir docs types:check — clean
• pnpm --dir docs lint — exit 0 (2 known scaffold-CSS warnings)
• pnpm --dir docs build — clean static export to docs/out/
• grep -rn "docs-next\|zensical\|grigori\.in" across repo — empty (only docs: commit-type mention in semantic-pr.yml remains, which is correct)
• CI on this PR: Deploy Docs job runs types:check + lint + build against the renamed docs/ path
• Manual smoke after merge: visit https://docs.byteveda.org/taskito — should show the new Fumadocs landing
• Manual smoke: hit https://docs.byteveda.org/taskito/llms.txt — should return the IA index

Phases summary (full migration arc)

Phase	PR	What
1	#110→#111	Scaffold (Docusaurus first, then Fumadocs after the GH-Pages re-evaluation)
2	#113	Port Home + Getting Started + Architecture
3	#115	Port API Reference
4 a–j	#116–#125	Port all 71 guide / integrations / examples pages
5	#126	Landing page polish
6	(built into Fumadocs scaffold — /llms.txt, /llms-full.txt, /raw/... routes shipped from day one)
7	this PR	Cutover

After merge, v0.12 is shippable. Tag + PyPI/crates.io publish are deliberate manual steps.