Fourth PR of the stack adding hostname-allowlisted network access to
agent terminal sandboxing. Stacked on the `http_proxy` crate PRs
(#59217, #59222, #59223) — review/merge those first; this PR's diff is
only the sandbox change.

Replaces the `allow_network: bool` field on the macOS Seatbelt
`SandboxPermissions` with a `NetworkAccess` enum: `None` (block all
egress), `All` (the previous `allow_network: true` behavior), and
`LocalhostPort(u16)`, which confines a sandboxed command to a single
loopback port. The new variant emits a Seatbelt rule permitting outbound
TCP only to `localhost:<port>` and isn't constructed yet; it exists to
pin sandboxed commands to the in-process network proxy in a later PR.
The sole consumer maps its existing boolean to `All`/`None`, so behavior
is unchanged.

Release Notes:

- N/A