render.py (4): variable substitution, StrictUndefined raises on missing
keys, evaluate_predicate coerces to bool, render_value walks dicts/lists
and renders string leaves.
filters.py (4): slugify produces URL-safe output, date formats datetime
with strftime, date(None) → "" so templates can write
{{ inputs.last_fired_at | date }} on first run, date(str) passes through.
environment.py (4): the sandbox boundary — disallowed Jinja built-ins
(e.g. pprint) raise, and the finalize hook coerces non-string outputs
to predictable wire shapes (datetime → ISO, None → "", dict → JSON).
context.py (1): build_run_context exposes {run, inputs, steps} with the
exact shape every plan template body relies on.
13 tests total, all pure unit.
