docs: widen sidebar + fix Claude Code install method + clarify MCP verification

[markshust]

Small docs/CSS follow-ups after the marko/devai landing.

Changes

Sidebar width — docs/src/styles/custom.css overrides Starlight's --sl-sidebar-width from the default 18.75rem to 19.75rem. Starlight derives the whole layout from this single variable (--sl-content-width is independent, and the print stylesheet keeps its own 0px override), so no other change is needed.

Claude Code install method — ai-assisted-development/agents/claude-code.md was telling users to install via npm install -g @anthropic-ai/claude-code and authenticate with claude auth login. Both are stale:

• The current recommended method is the native installer: curl -fsSL https://claude.ai/install.sh | bash (npm still works but is no longer preferred; the native install auto-updates). Source: Claude Code setup docs.
• There is no claude auth login command — first run opens a browser sign-in; /login manages credentials in-session. claude doctor verifies the install.

.claude/settings.json explanation — expanded the two key bullets (extraKnownMarketplaces.marko, enabledPlugins) with what each key is, the local-vs-GitHub source shapes, and how Claude Code consumes them on folder-trust (and that devai only touches *@marko entries).

Verification checklist — Claude Code's MCP server comes from the marko-mcp@marko plugin, not claude mcp add (the agent actively removes legacy claude mcp registrations). Changed the check from claude mcp list to .claude/settings.json + /mcp.

Verification

• cd docs && npm run build → 140 pages, clean.
• composer test (docs-markdown) → 12 passed.

Context: this is also the point at which I functionally verified the servers end-to-end (booted mcp:serve and confirmed list_modules / find_event_observers return correct results; booted lsp:serve and confirmed a real config-key completion round-trip). The doc fixes above came out of that exercise.