docs: add Status & Capacity and Runtime (devkitd) pages - #354

Verified finding

status, capacity, and the devkitd daemon shipped across 0.59–0.69 but have zero documentation (grep of web/content/docs: only unrelated channel --daemon hits).

• Reality: packages/cli/src/commands/{status,capacity,daemon}.ts; capacity supports 6 providers (codex, zai/z.ai, openai, anthropic, claude, devin); daemon auto-spawns on ~/.ai-devkit/daemon.sock, logs to daemon.log, resolves binary via DEVKITD_BIN/platform package/cargo; daemon install writes a systemd --user unit (Linux).

What changed (updated: split per review)

Originally one 15-runtime-and-capacity page; now split into two dedicated pages:

• 15-capacity.md — Status & Capacity: status readiness report, capacity quota command (6 providers, --json), and picking agent types by provider headroom
• 16-runtime.md — Runtime (devkitd): what the daemon is (Rust, auto-spawn), socket/log/binary resolution, daemon status/start/stop/logs/install, troubleshooting, and the "Upgrading to 0.69" note

Validation

• npm run build in web/: both pages prerender (/docs/15-capacity, /docs/16-runtime)
• npm run lint at root: pass

Generated with Devin

technical-writer review pass: ran; clarified that capacity reuses each provider's local login (unauthenticated providers report as such).