feat(host-agent): Phase 1c — SteamCMD update + jailed file manager

steam_update func runs SteamCMD per game (rust/conan/soulmask app-ids;
dune rejected), streaming stdout to {instance}.steam_status. Jailed
file manager on {instance}.files.cmd: list/read/write/delete/rename/
mkdir/mkfile/move/copy, all confined to instance root via two-stage
lexical-normalize + canonicalize (defeats ../ traversal AND symlink
escape — incl chained symlinks). Replaces the Go agent's UNJAILED
legacy files API (retired, not ported). 5MiB read cap.

42/42 tests green: 24 filemanager incl 7 jail-escape attempts
(dotdot, deep dotdot, absolute, symlink-inside, direct symlink,
chained symlink), 5 steamcmd app-id (cfg-gated win/linux soulmask).
Jail logic reviewed line-by-line: Path::starts_with is component-wise
(no sibling-prefix bypass), non-existent suffix components can't be
symlinks, leading .. normalizes to / and fails the prefix check.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/src/config.rs

@@ -11,6 +11,7 @@ use std::collections::HashSet;
 use std::path::{Path, PathBuf};
 
 use crate::rcon::RconConfig;
+use crate::steamcmd::SteamcmdConfig;
 
 /// Instance ids share the NATS subject namespace with host-level segments.
 const RESERVED_INSTANCE_IDS: &[&str] = &["host", "cmd", "files", "update", "agent"];
@@ -65,6 +66,10 @@ pub struct InstanceConfig {
     /// Protocol defaults to WebRcon for rust, Source for conan/soulmask.
     #[serde(default)]
     pub rcon: Option<RconConfig>,
+    /// SteamCMD update settings.  Absent = defaults apply (steamcmd on PATH,
+    /// validate = false).
+    #[serde(default)]
+    pub steamcmd: Option<SteamcmdConfig>,
 }
 
 impl InstanceConfig {