Two review follow-ups on the agent command tree.
Artifact download (data integrity): fetchArtifactURL now reads one byte past
maxArtifactBytes and refuses a body over the cap with a typed error, instead of
letting io.LimitReader silently truncate a >256 MiB artifact to a corrupt,
partial file that reported success (exit 0). The LimitEnforced test is inverted
to assert the rejection.
Capabilities: the single multi_turn card bit is replaced by three independent
capabilities — context_list / context_get / context_delete — each derived from
its own wired hook (ListContexts / GetContext / DeleteContext). One bit could
not honestly represent three separately-deliverable verbs: a provider wiring
ListContexts but not DeleteContext advertised multi_turn=true while `context
delete` failed claiming multi_turn=false. Each context verb now gates on and
reports its own capability. Card schema, capability matrix, the three context
gates, tests, and the lark-agent skill docs are updated in lockstep.
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