`validate_container_name`'s doc comment read "check it maps to a
hive-managed container". The body checked characters. Nothing in it
ever tested hive-managed-ness, so at the privsep boundary it asserted
a security property that was not there -- the kind of comment a future
reader relies on when deciding a name is already confined.
The confinement is real, it just lives elsewhere: all five callers
immediately apply `container_system_name`, so a caller naming
`hive-forge` reaches `h-hive-forge`, not the forge. That guarantee now
sits on `container_system_name`, which is the function that provides
it.
The `SIBLING_CONTAINERS` early-return in that validator was dead in
every path, not merely unused today: `validate_name_chars` accepts
`[a-z0-9-]`, so all three sibling names pass the fallthrough
identically -- both arms, same result. Infra containers reach the
lifecycle verbs through `ControlInfraContainer` and the
`InfraContainer` enum, which carries no free-form name at all, so no
sibling name reaches these functions. No test pinned the branch.
With the dead branch gone the function was character-for-character
`validate_agent_name`, so it is deleted and its five call sites use
that directly. `SIBLING_CONTAINERS` stays imported --
`validate_container_system_name` still needs it, and there the same
early-return IS load-bearing: without it an unprefixed sibling name
hits the bail.
No behaviour change. Gated: fmt, clippy -D warnings, 21 tests, rustdoc
under CI's docs-rustdoc lints (the two new intra-doc links resolve),
push-lints.