v0.5.2: shell_bg / remote_shell_bg — background launch tools

server.lua gains a shell_bg tool that launches a detached command via
setsid + nohup + stdio-redirect + &, returns immediately with PID and
log path. Linux-only for MVP (Windows Start-Process equivalent TBD).

hub.lua gains remote_shell_bg, forwarding to backend shell_bg. lmcp-only,
no ssh fallback — fallback for fire-and-forget is semantically murky.

Addresses the 'how do I launch a daemon over lmcp without the sentinel-
file wrapper blocking forever' question. Existing remote_shell keeps
its current synchronous-with-timeout behaviour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server.lua

@@ -170,6 +170,49 @@ server:tool("shell", "Execute a shell command.", {
     return run(cmd, a.timeout or 120)
 end)
 
+server:tool("shell_bg",
+    "Fire-and-forget shell command (Linux-only). Fully detaches via setsid+nohup+stdio-redirect and returns immediately with PID and log path. Use for daemons that must outlive the lmcp request.",
+    {
+        type = "object",
+        properties = {
+            command = { type = "string", description = "Shell command to launch" },
+            cwd     = { type = "string", description = "Working directory" },
+            log     = { type = "string", description = "Log file (stdout+stderr). Default: /tmp/lmcp-bg-<ts>-<rand>.log" },
+        },
+        required = { "command" },
+    },
+    function(a)
+        if WINDOWS then
+            return "Error: shell_bg is Linux-only (Windows Start-Process equivalent TBD)"
+        end
+        if type(a.command) ~= "string" or a.command == "" then
+            return "Error: command required"
+        end
+        local log = a.log
+        if not log or log == "" then
+            log = string.format("/tmp/lmcp-bg-%d-%d.log", os.time(), math.random(1000, 9999))
+        end
+        local pid_file = log .. ".pid"
+        local inner = a.command
+        if a.cwd and a.cwd ~= "" then
+            inner = "cd '" .. a.cwd:gsub("'", "'\\''") .. "' && " .. inner
+        end
+        local sq = function(s) return "'" .. s:gsub("'", "'\\''") .. "'" end
+        local full = string.format(
+            "setsid nohup sh -c %s </dev/null >%s 2>&1 & echo $! > %s",
+            sq(inner), sq(log), sq(pid_file)
+        )
+        os.execute(full)
+        local f = io.open(pid_file, 'r')
+        local pid = "?"
+        if f then
+            pid = (f:read('*a') or ""):match("(%d+)") or "?"
+            f:close()
+            os.remove(pid_file)
+        end
+        return string.format("launched pid=%s log=%s", pid, log)
+    end)
+
 server:tool("read_file", "Read a file.", {
     type = "object",
     properties = { path = { type = "string" } },