docs: add comprehensive developer guide (mdBook)

Add a browsable mdBook developer guide under docs/dev-guide/ covering the
full PiCloud v1.1.9 surface for developers building on the platform:

- Guide: introduction, 10-minute quickstart, core concepts, writing scripts
- Tutorials: 5 end-to-end example apps (URL shortener, webhook receiver,
  TODO API with auth, scheduled report, file-upload service) + feature matrix
- SDK reference: kv/docs/files, pubsub/queue, http, email, users, secrets,
  invoke/retry/dead_letters, stdlib, ctx & events
- HTTP API reference: every /api/v1 endpoint, auth, and capability
- CLI reference: the `pic` client + the server admin recovery command
- Config: every PICLOUD_* env var, capabilities & roles
- Deployment: Docker Compose, bare binary, Caddy/TLS, production checklist
- Operations: security, best practices, troubleshooting

Every example was built and run against a live instance; SDK signatures,
endpoint paths, and env vars were verified against source. Build the site
with `mdbook serve docs/dev-guide`. A pointer is added to README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/dev-guide/.gitignore

@@ -0,0 +1,2 @@
+# mdBook build output — generated by `mdbook build`, not source.
+book/