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/agent.example.toml

@@ -46,6 +46,16 @@ password = "changeme"
 # password = "changeme"
 # # kind = "source"  # inferred automatically for soulmask
 
+# SteamCMD update settings — optional sub-table for any instance.
+# Absent = defaults: steamcmd binary resolved via PATH, validate = false.
+#
+# [instance.steamcmd]
+# steamcmd_path = "/opt/steamcmd/steamcmd.sh"  # omit to use PATH
+# validate = true                               # enable file-hash check pass
+#
+# Dune instances do not use SteamCMD (Docker images); the steam_update func
+# will return a clear error if invoked on a dune instance.
+
 [prober]
 interval_seconds = 300