fix(skills): download roster members' skills, not just the coordinator's

[Chirag6722]

The problem

download_session_skills collects skills from session.agent.skills only:

for skill in session.agent.skills:

In a multiagent session the roster members' skills live at session.agent.multiagent.agents[*].skills, and were never fetched. Since subagent threads share the coordinator's container filesystem, a self-hosted worker ran subagents with their skills absent from {workdir}/skills/.

Nothing raised. The thread behaved as though the skill were not attached, so the only symptom was degraded output — which is what makes it worth fixing rather than merely wrong.

grep -rn multiagent src/anthropic/lib/ still returns nothing, so this is the first place the lib layer learns the roster exists.

The fix

A _session_skills helper yields the coordinator's skills followed by each roster member's, and download_session_skills iterates that instead.

Two details drove the shape of it:

Roster entries are a discriminated union. BetaManagedAgentsSessionMultiagentCoordinator.agents is List[Agent] where Agent = Union[BetaManagedAgentsSessionThreadAgent, BetaManagedAgentsAdvisor]. Thread agents carry skills; advisors have only model and type. So the entries are filtered on agent.type == "agent" rather than by probing for the attribute — the obvious for a in agents: a.skills raises AttributeError on any roster containing an advisor.

Shared skills are yielded once. The download loop rmtrees each destination before extracting, so a skill attached to both the coordinator and a member would tear down and redo work already completed. Dedupe is keyed on (skill_id, version), so the same skill pinned to two versions is still fetched twice — those are different archives.

Sessions with no multiagent, or with an empty roster, are unchanged.

Tests

Six cases in tests/lib/tools/test_skills.py, matching the file's existing style of exercising the private helpers directly:

• no roster — unchanged single-agent behaviour
• roster members' skills are included
• an advisor in the roster is skipped rather than raising
• a skill shared by coordinator and member is yielded once
• the same skill at two versions is not collapsed
• an empty roster behaves like no roster

tests/lib/tools/test_skills.py -k session_skills   ->  6 passed

Full file on this branch: 7 failed, 19 passed. On unmodified main the same file is 7 failed, 13 passed — the seven are pre-existing failures on Windows in the Unix file-mode tests (test_zip_preserves_executable_bit and friends, asserting 0o755 where Windows reports 0o666), unrelated to this change. I ran them with --noconftest because tests/conftest.py imports http_snapshot, which I could not install.

ruff check and ruff format --check are clean on both files.

Closes #1870