fix(packaging): move templates/ into decnet/ package so they ship with pip install

The docker build contexts and syslog_bridge.py lived at repo root, which
meant setuptools (include = ["decnet*"]) never shipped them. Agents
installed via `pip install $RELEASE_DIR` got site-packages/decnet/** but no
templates/, so every deploy blew up in deployer._sync_logging_helper with
FileNotFoundError on templates/syslog_bridge.py.

Move templates/ -> decnet/templates/ and declare it as setuptools
package-data. Path resolutions in services/*.py and engine/deployer.py drop
one .parent since templates now lives beside the code. Test fixtures,
bandit exclude path, and coverage omit glob updated to match.

decnet/services/smtp_relay.py

@@ -4,7 +4,7 @@ from decnet.services.base import BaseService
 
 # Reuses the same template as the smtp service — only difference is
 # SMTP_OPEN_RELAY=1 in the environment, which enables the open relay persona.
-_TEMPLATES_DIR = Path(__file__).parent.parent.parent / "templates" / "smtp"
+_TEMPLATES_DIR = Path(__file__).parent.parent / "templates" / "smtp"
 
 
 class SMTPRelayService(BaseService):