refactor(host): tokens live in a file under the data root (U0e)

D-63: the 0700 data root is the trust boundary already (I-1: hidden, not
locked), so the OS keyring added no wall and cost headless servers a D-Bus
session, CI a knob and every OS its own store. The file keeper is the only
store: run/tokens/, 0600, written atomically. The three keyring stores, the
file_keyring knob and their tests are removed; an unreadable token file is an
error, never no token.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-host/src/lib.rs

@@ -8,7 +8,7 @@
 //! [`host::run`] (`wanderbooks host`), [`bridge::run`] (`wanderbooks
 //! mcp`), and the owner's commands in [`commands`] (`serve`, `close`,
 //! `books`, `token`, `connect`, `doctor`, `passphrase`, and what the
-//! viewer's Books menu does). Agents' tokens are kept in the OS keyring
+//! viewer's Books menu does). Agents' tokens are kept in files under the data root
 //! ([`tokens`]).
 
 #[cfg(unix)]