feat: add some domain knowledge for agents

AGENTS.md

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+# juju-controller Agent Rules Index
+
+Ensure that the following documents have been read:
+
+ - [Juju Hook Lifecycle](https://documentation.ubuntu.com/juju/3.6/reference/hook/)
+ - [Operator Framework](https://documentation.ubuntu.com/ops/latest/reference/)
+
+If guidance conflicts, Juju Hook Lifecycle rules take precedence.
+
+## Setup
+
+Create and activate a virtualenv, and install the development requirements:
+
+```
+virtualenv -p python3 venv
+source venv/bin/activate
+uv pip install ".[dev]"
+```
+
+Install `astral-uv` using snaps:
+
+```
+sudo snap install astral-uv --classic
+```
+
+## Build
+
+Install `charmcraft` using snaps:
+
+```
+sudo snap install charmcraft --classic
+```
+
+Then run charmcraft pack:
+
+```
+charmcraft pack -v
+```
+
+## Updating libs
+
+```
+charmcraft fetch-libs
+```
+
+## Running Tests
+
+- `./run_tests`
+