Remove redundant dotenv dependency (#1844)

`dotenv` was added in 605bca19 together with the `from dotenv import
load_dotenv` call in `src/serena/agno.py`, six days after `python-dotenv` had
already been added in 276f6a84. The `dotenv` module that the code imports is
provided by `python-dotenv`. The separately named `dotenv` distribution on PyPI
is an unrelated, deprecated project by another author, republished as a
metadata-only stub whose only content is `Requires-Dist: python-dotenv`; its
wheel ships no Python module at all, so the dependency has never contributed
anything at runtime.

The redundancy survived the bulk pinning pass in 45cd8ca1, which mechanically
rewrote `dotenv>=0.9.9` to `dotenv==0.9.9`.

Dropping it removes a third-party PyPI name from the set of distributions
trusted at install time, with no behavioural change. Verified that `from dotenv
import load_dotenv` resolves with only `python-dotenv==1.2.2` installed.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -28,7 +28,6 @@ dependencies = [
   "pyyaml==6.0.2",
   "ruamel.yaml==0.18.14",
   "jinja2==3.1.6",
-  "dotenv==0.9.9",
   "pathspec==0.12.1",
   "psutil==7.0.0",
   "docstring_parser==0.17.0",