test(presets): silence expected UserWarnings in self-test composition… (#2373)

* test(presets): silence expected UserWarnings in self-test composition tests

The self-test preset that ships with the repo provides a wrap-strategy
command (speckit.wrap-test) intentionally without a corresponding core
base layer, exercising the 'no base layer' branch of
_reconcile_composed_commands().

Eighteen tests across TestSelfTestPreset and TestPresetSkills install
this preset and trigger an expected UserWarning. Running the suite with
-W error::UserWarning surfaces them as test noise that could obscure
unrelated warnings.

Add class-level pytest.mark.filterwarnings filters to acknowledge the
two known messages ('Cannot compose command speckit.wrap-test' and
'Post-install reconciliation failed for self-test') so other UserWarning
sources still propagate normally.

Fixes #2363

* test(presets): scope filterwarnings to UserWarning category

Address Copilot review on #2373: the previous filterwarnings entries
omitted the warning category, so any warning class with a matching
message would have been silenced. Append :UserWarning to the four
filters so only the deliberately-emitted UserWarnings from
_reconcile_composed_commands() are ignored.

* test(presets): narrow self-test warning filter to install helper only

Address Copilot feedback: the class-level @pytest.mark.filterwarnings on
TestPresetSkills was too broad. The 'Post-install reconciliation failed'
filter could mask real reconciliation regressions, since that warning is
only emitted when _reconcile_composed_commands/_reconcile_skills raises.

Tests in TestPresetSkills already call install_self_test_preset(), which
scopes a narrow filter to the expected wrap-strategy 'Cannot compose'
warning. The class-level filters are redundant for those calls and unsafe
elsewhere, so they are removed.

* test(presets): align TestSelfTestPreset docstring with helper-based filtering

Address Copilot feedback: docstring referred to 'filters above', but the
fix uses warnings.filterwarnings inside install_self_test_preset rather
than class-level decorators. Updated the docstring to describe the actual
mechanism.

* test(presets): remove extra blank line between helper and class (PEP 8)

Address Copilot feedback: PEP 8 expects two blank lines between top-level
definitions; reduce the three blank lines between install_self_test_preset
and TestSelfTestPreset to two.