Local Dev: working examples for rule authors

[harry-anderson]

Adds 9 working example rules to the local-dev environment covering the plaid-stl API surface. All examples compile to WASM and are wired into the docker-compose setup from #272.

Examples

Example	What it teaches	Difficulty
echo-json	Parse JSON webhook, return response	Beginner
counter	Persistent storage vs in-memory cache	Beginner
cron-heartbeat	Timer-triggered rule	Beginner
webhook-router	Chain rules via logback	Beginner
rate-limiter	Cache-based rate limiting	Intermediate
request-logger	HTTP headers, query params	Intermediate
todo-api	Full CRUD API with storage	Intermediate
http-proxy	Outbound HTTP via named requests	Advanced
error-handling	thiserror + set_error_context	Advanced

Also includes a rules README with curl test commands and an "Anatomy of a Rule" guide for new authors.

Depends on #272.

[obelisk]

We should merge this before #272 right?

[harry-anderson]

@obelisk yep, i just didnt want to have mega size PR 🤣