release: 0.20.0 — drop Node 20, MCP fixes, README overhaul

[esengine]

Release commit for 0.20.0. Bumps package.json version and adds the corresponding CHANGELOG.md entry. No code-path changes in this PR — those landed in #91, #96, #98, #99, #102.

Highlights

Breaking: drops Node 20 support (engines.node >= 22). Node 20 reached EOL on 2026-04-30.

Fixes:

• reasonix code now bridges config.mcp servers (fix(code): bridge config.mcp servers in code mode (closes #90) #91)
• kebab-case MCP spec names parse correctly (fix(mcp): allow hyphens in MCP spec name prefix regex #96)

Docs / hygiene:

• README design-token SVG suite — hero terminal, hero stats, feature grid (feat(readme): visual polish — hero terminal · stats · feature grid #102)
• Designer-pass on hero block, comparison table trimmed, contributor avatars, COC, SECURITY (feat(readme): visual polish — hero terminal · stats · feature grid #102)
• Design mockups moved into docs/design/, README assets into docs/assets/ (feat(readme): visual polish — hero terminal · stats · feature grid #102)
• Dynamic good-first-issue label link (docs(readme): replace hardcoded good-first-issue list with label link #99)
• Drop "DeepSeek free credit on signup" claim (feat(readme): visual polish — hero terminal · stats · feature grid #102)

After merge

• Tag v0.20.0 on the squash-merge commit
• Push tag
• npm publish