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/Cargo.toml

@@ -30,6 +30,9 @@ tokio-tungstenite = "0.24"
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
+[dev-dependencies]
+tempfile = "3"
+
 # Size-optimized release: single static binary living next to RAM-heavy game
 # servers. Panic stays 'unwind' so a panicking task surfaces through its
 # JoinHandle instead of killing the whole agent.