hive-sh4re: split inbox, container, journal, and schedule wire shapes into their own modules

Closes the #3110 split — lib.rs is now just the crate doc comment and
the pub mod list.

journal.rs's new doc comment fixes a pre-existing bug: the old
JournalPriority doc text in lib.rs was actually half Capability's doc
(a leftover from an earlier reorder that moved the code but not the
comment above it).

hive-agent-mcp/src/mcp/args.rs

@@ -334,7 +334,7 @@ pub struct GetHostJournalArgs {
     pub lines: Option<u32>,
     /// Minimum syslog priority level.
     #[serde(default)]
-    pub priority: Option<hive_sh4re::JournalPriority>,
+    pub priority: Option<hive_sh4re::journal::JournalPriority>,
     /// Regex to match against log message fields (journalctl --grep).
     #[serde(default)]
     pub grep: Option<String>,