fix(harness): EnvironmentContext.today_date returns naive datetime in timezone fallback path

[caoergou]

Problem

EnvironmentContext.today_date is documented as returning a timezone-aware datetime:

• The EnvironmentContext type's docstring states: "Timezone-aware datetime on the computer"
• The comment in EnvironmentResolver.resolve() reads: "Parse into a timezone-aware datetime."

However, the fallback path in environment.py (line 81) silently produces a naive datetime when strptime with %z fails:

# environment.py  line 80-81
except ValueError:
    now = datetime.strptime(raw_dt[:19], "%Y-%m-%dT%H:%M:%S")  # noqa: DTZ007

The # noqa: DTZ007 suppression is itself a signal that the linter flagged this as a timezone-unaware datetime. The existing test even asserts the broken contract explicitly:

# test_environment.py  line 84
assert env.today_date.tzinfo is None   # contradicts the documented "timezone-aware" guarantee

Impact

Any downstream code that treats today_date.tzinfo as reliably set — such as timezone-aware arithmetic or comparisons — will raise a TypeError on hosts where the shell's date output lacks a numeric UTC offset (some containers, minimal environments, or locale edge cases).

Proposed Fix

In the except ValueError fallback, attach UTC explicitly so the contract is always satisfied:

from datetime import UTC, datetime

# ...
except ValueError:
    now = datetime.strptime(raw_dt[:19], "%Y-%m-%dT%H:%M:%S").replace(tzinfo=UTC)

This removes the need for # noqa: DTZ007 and brings the test assertion in line with the documented contract:

assert env.today_date.tzinfo is not None

The change is minimal (1 import, 1 line, 1 test assertion) and does not affect the happy path.