Document the HEALTH_CHECK_SMTP_PORT environment variable #42

What to build

The health check's email notifier accepts an optional SMTP port override, read
from the environment as HEALTH_CHECK_SMTP_PORT. When it is unset, the notifier
leaves the port to the mail transport's own default.

The README's environment variable table documents the host, user and password
for that notifier, but not the port. An operator whose SMTP relay listens
anywhere other than the default port therefore has no way to discover that the
knob exists short of reading the source.

Add a row for it to that table, alongside the other health check SMTP entries,
following the table's existing columns and wording conventions. It is optional,
and it has no default of its own.

Acceptance

• The README's environment variable table contains a row for HEALTH_CHECK_SMTP_PORT.
• The row describes it as the SMTP port for unhealthy notification emails and marks it optional, with column values consistent with the neighbouring host, user and password rows.
• No other environment variable row is added, removed or reworded.

Effort: workspace/2026-09-13-dcc-agent-tooling