docs(#2862): document the snapshot store, drop the dedicated option

mara: the option was the wrong shape for the concern. "this host runs
nothing else" is a deployment expectation, not something a module
should assert about its own host — and asserting it made co-location
look like a config toggle rather than what it is.

Replaced with docs/snapshot-store.md, which the module had no docs
page at all before: enabling it, why the mesh is the authentication
(cryptokey routing already binds source address to pubkey, so certs
would authenticate the same fact twice and add an expiry), why the
destination is keyed per agent (a per-hive prefix splits an agent's
chain the first time it migrates), what the sender may and may not
choose, why the firewall rule is interface-scoped, what a snapshot
does and does not contain, and what the pull side still needs.

The dedicated-host expectation is stated there as an operational
assumption with its own failure mode — true on day one, quietly false
the day someone notices the box has spare disk — rather than as an
assertion someone flips to false to make the build proceed.

Linked from CLAUDE.md's reading paths.

CLAUDE.md

@@ -165,6 +165,10 @@ read them à la carte.
 - **"How do I connect two hives into a swarm? How do I declare peer
   hives and configure TLS trust?"** →
   [`docs/swarm.md`](docs/swarm.md).
+- **"Where do agent snapshots go? How does the swarm's `btrfs receive`
+  endpoint authenticate a pushing hive, and what does a snapshot
+  actually contain?"** →
+  [`docs/snapshot-store.md`](docs/snapshot-store.md).
 - **"How does the rebuild queue work? What are queue kinds and sources?"** →
   [`docs/coordinator.md`](docs/coordinator.md).
 - **"How does the CI runner work? What's the auto-registration flow?"** →