Add web optional dependency and update docs

Flask is now declared under [project.optional-dependencies] as the
`web` extra, so `pip install osm-geojson[web]` installs both the CLI
and Flask. The `dev` extra includes Flask too so the test/dev venv
gets everything. Update README and AGENTS.md accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

AGENTS.md

@@ -81,10 +81,10 @@ cd web
 flask --app app.py run
 ```
 
-The web dependencies (Flask) are not in `pyproject.toml`; install separately:
+Install the package with the `web` extra to include Flask:
 
 ```
-pip install flask
+pip install -e ".[web]"
 ```
 
 ## Type checking

README.md

@@ -88,8 +88,8 @@ routes as GeoJSON.
 #### Running the dev server
 
 ```
-cd web
-flask --app app.py run
+pip install -e ".[web]"
+flask --app web/app.py run
 ```
 
 Open `http://127.0.0.1:5000`.

pyproject.toml

@@ -15,11 +15,15 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+web = [
+    "flask>=3.0",
+]
 dev = [
     "mypy",
     "pytest",
     "responses",
     "types-requests",
+    "flask>=3.0",
 ]
 
 [project.scripts]