[AIGTWY-4977] Cover managed skills in CUJ 2, 3 and 6

[david-siqi-liu]

The CUJ plan's skills checks for CUJ 2, 3 and 6 were skipped because Default Storage rejects skill uploads. With skill fixtures on a schema backed by customer S3 storage, the journeys can now cover them.

Changes:

• CUJ 2 (skills.names = ["ug_e2e.skills.fixture-summary"]):
  • only that bundle downloads, byte-identical to the live fixture;
  • both agents list it in /skills, and the schema's other skills are absent;
  • both agents invoke it and answer with a token that exists only in the bundle's reference file.
  • _assert_cuj2_config now expects the skills selector.
• CUJ 3 (skills.unity_catalog_location = "ug_e2e.skills"):
  • every skill in the schema downloads and is listed;
  • the ug_e2e.other_skills decoy stays absent;
  • ug status counts the skills per agent;
  • both agents invoke a downloaded skill.
• CUJ 6:
  • Phase A publishes the named skill. It downloads once to both skill roots and is unchanged by a repeat configure.
  • Phase A runs no terminal launch: one would make ug own /etc, and phase B's headless configure would then refuse.
  • Phases B and C drop the selector, and the skill disappears from disk and from each agent's /skills, including after relaunch.
  • The hand-authored user skill must render in the same screen, so the absence checks can't pass on an unloaded list.
  • The user's Claude settings are seeded with the theme the Claude TUI saves on first launch, so only ug's writes show as a diff.
• Shared helpers live in tests/e2e_cuj/helpers/skills.py. The tests/README.md coverage row is updated.
• UC skill names reject underscores, so the plan's fixture_summary is fixture-summary.

One-time workspace setup, done outside the tests on 2026-10-07:

• CUJ 6 metastore (ca-central-1):
  • ug_e2e.skills sits on S3-backed storage;
  • the CUJ SP has READ_VOLUME on it, which skill_authz.jsonnet requires for skill reads and downloads;
  • workspace A publishes the Phase A skills selector.
• CUJ 2/3 metastore (us-east-1):
  • the same S3-backed storage credential and an external location;
  • ug_e2e.skills and ug_e2e.other_skills recreated on that storage, holding the four fixture skills;
  • CUJ 2 publishes skills.names, and CUJ 3 already published unity_catalog_location = ug_e2e.skills.

This overlaps Mason's open #1014 and #1015 in test_cuj2_mps_mcp.py, where this PR only adds one key to _assert_cuj2_config.

This pull request and its description were written by Isaac.