feat(deploy): self-host recipe, the enforced isolation flag, and the tunnel's door (P9a)

The books can live on a machine of the owner's own (plan 3.8, 8.2 P9a,
D-51). Three things make that honest instead of nominal:

- `--isolation same_user|enforced` sets `server::Isolation`, carried
  through `Served` and `BooksServer` to `books.start`. Locally the books
  stay `same_user (advisory)`; under the self-host unit the server is told
  `enforced` and reports `service_user (enforced)`. The server never
  guesses, and the literal it used to report is gone.
- `--http [ADDR]` binds the same MCP router on loopback (default
  127.0.0.1:7411) beside the Unix sockets, before the ready line, for the
  TLS proxy or tunnel in front of the server. It is the same bearer door
  and the same graceful close; a failed bind is reported and is not fatal
  (3.9). `serve` carries both flags through the host, so the system unit
  can ask for them once.

`deploy/wanderbooks-server.service` is the unit: a dedicated account, a
0700 data root, `ProtectSystem=strict`, no seccomp or Landlock (D-32), and
loopback-only networking. `deploy/README.md` is the recipe: install, setup,
move the books by `books adopt` with its head check (D-31: a copied book
is the transport), mint a bearer token, put Caddy or Tailscale Funnel in
front of the loopback door, and point Claude Code and Codex at it.

The two-account CI job (`.github/workflows/two-user.yml`) makes the
deployment and runs `crates/ledger-cli/tests/two_user.rs` as another
account: `agent_uid_cannot_read_tenant` proves the tenant root and its
journal are refused, then reads, writes, uploads, files, exports and
verifies over HTTP with the bearer (in a second book, so the moved books'
head is not advanced); `self_host_adopt_same_head` compares the moved
books' head and event count with the source clone's.
`isolation_label_is_enforced_under_the_unit` runs everywhere, including
the unit-file read that keeps the recipe from drifting.

Accounting reason for the shape: nothing here changes an entry, a report
or a figure. It changes who can reach them: the move is lossless and
verifiable at the head, and the boundary is the operating system's, not a
promise in our text. No journal code, kernel function, Kani harness or
Lean step is touched, so `just prove` is not required.

No go-ahead needed: no new journal feature or event; the live books are
unaffected.

crates/ledger-mcp/src/tools/kept.rs

@@ -266,9 +266,11 @@ pub struct StartOut {
     pub format_upgrade: Option<FormatUpgradeOut>,
     /// What to do next, in order.
     pub next_steps: Vec<NextStepOut>,
-    /// How the books are kept from agents: "same_user (advisory)", hidden
-    /// from them in the human's own account, not locked against them
-    /// (plan D-32): reach them only through these tools.
+    /// How the books are kept from agents: "same_user (advisory)" locally
+    /// (hidden from them in the human's own account, not locked against
+    /// them: plan D-32), or "service_user (enforced)" when the books run
+    /// on the owner's own machine as a dedicated account (plan 3.8, P9a).
+    /// Either way, reach them only through these tools.
     pub isolation: String,
 }
 
@@ -921,7 +923,7 @@ impl BooksServer {
                 how: "only the owner, with the owner passphrase: in the app (it asks when it opens the books) or `wanderbooks books upgrade ID` at a terminal; a verified backup with the keys is made first. No tool can.".to_string(),
             }),
             next_steps,
-            isolation: "same_user (advisory)".to_string(),
+            isolation: self.isolation.label().to_string(),
         };
         // Fit (INVARIANT 4): whole lists if they fit; else summaries go,
         // then the lists shorten from the end, each saying how many more.