MAINT: Add DEFAULT strategy alias to airt.cyber scenario

[varunj-msft]

Description

Part of the Standardizing Scenarios effort. This brings airt.cyber in line with
the convention the other scenarios follow: a DEFAULT strategy aggregate that drives the
default run, with ALL kept for the exhaustive run.

Cyber only has one technique today (red_teaming), so DEFAULT expands to exactly the same
thing as ALL — this isn't a behavior change, it's added for cross-scenario convention parity
so --strategies default works uniformly everywhere.

One wrinkle worth calling out: red_teaming isn't tagged default in the shared technique
catalog, so the usual TagQuery.any_of("default") would produce an empty aggregate and collapse
the default run to baseline-only. Cyber's DEFAULT matches core instead, which selects the
curated technique without tagging red_teaming globally (that would leak it into other
scenarios' default runs). There's a comment on _CYBER_TECHNIQUE_NAMES noting that adding
another core-tagged technique would pull it into DEFAULT.

Not a breaking change: the default run produces the same atomic attacks as before (same
red_teaming + baseline), so output is byte-identical and --resume stays compatible. No
VERSION bump needed.

Tests and Documentation

Tests (tests/unit/scenario/airt/test_cyber.py):

• default strategy is now DEFAULT (was ALL)
• DEFAULT expands to red_teaming and is non-empty (guards against the empty-aggregate footgun)
• DEFAULT is a subset of ALL
• updated existing assertions/docstrings that referenced ALL

All 25 cyber tests pass; full airt scenario suite (164) green. ruff, ruff format, and ty clean.

Documentation (doc/scanner/airt.py + .ipynb):

• added DEFAULT to the Cyber "Available strategies" list
• ran jupytext to keep the .py and .ipynb in sync