fix: improve background process handling for agent tools (#23132)

## Problem

Models frequently use shell `&` instead of `run_in_background=true` when
starting long-running processes through `/agents`, causing them to die
shortly after starting. This happens because:

1. **No guidance in tool schema** — The `ExecuteArgs` struct had zero
`description` tags. The model saw `run_in_background: boolean
(optional)` with no explanation of when/why to use it.
2. **Shell `&` is silently broken** — `sh -c "command &"` forks the
process, the shell exits immediately, and the forked child becomes an
orphan not tracked by the process manager.
3. **No process group isolation** — The SSH subsystem sets `Setsid:
true` on spawned processes, but the agent process manager set no
`SysProcAttr` at all. Signals only hit the top-level `sh`, not child
processes.

## Investigation

Compared our implementation against **openai/codex** and **coder/mux**:

| Aspect | codex | mux | coder/coder (before) |
|--------|-------|-----|---------------------|
| Background flag | Yield/resume with `session_id` | `run_in_background`
with rich description | `run_in_background` with **no description** |
| `&` handling | `setsid()` + `killpg()` | `detached: true` +
`killProcessTree()` | **Nothing** — orphaned children escape |
| Process isolation | `setsid()` on every spawn | `set -m; nohup ...
setsid` for background | **No `SysProcAttr` at all** |
| Signal delivery | `killpg(pgid, sig)` — entire group | `kill -15
-\$pid` — negative PID | `proc.cmd.Process.Signal()` — **PID only** |

## Changes

### Fix 1: Add descriptions to `ExecuteArgs` (highest impact)
The model now sees explicit guidance: *"Use for long-running processes
like dev servers, file watchers, or builds. Do NOT use shell & — it will
not work correctly."*

### Fix 2: Update tool description
The top-level execute tool description now reinforces: *"Use
run_in_background=true for long-running processes. Never use shell '&'
for backgrounding."*

### Fix 3: Detect trailing `&` and auto-promote to background
Defense-in-depth: if the model still uses `command &`, we strip the `&`
and promote to `run_in_background=true` automatically. Correctly
distinguishes `&` from `&&`.

### Fix 4: Process group isolation (`Setpgid`)
New platform-specific files (`proc_other.go` / `proc_windows.go`)
following the same pattern as `agentssh/exec_other.go`. Every spawned
process gets its own process group.

### Fix 5: Process group signaling
`signal()` now uses `syscall.Kill(-pid, sig)` on Unix to signal the
entire process group, ensuring child processes from shell pipelines are
also cleaned up.

## Testing
All existing `agent/agentproc` tests pass. Both packages compile
cleanly.

agent/agentproc/process.go

@@ -113,6 +113,7 @@ func (m *manager) start(req workspacesdk.StartProcessRequest, chatID string) (*p
 		cmd.Dir = req.WorkDir
 	}
 	cmd.Stdin = nil
+	cmd.SysProcAttr = procSysProcAttr()
 
 	// WaitDelay ensures cmd.Wait returns promptly after
 	// the process is killed, even if child processes are
@@ -272,13 +273,15 @@ func (m *manager) signal(id string, sig string) error {
 
 	switch sig {
 	case "kill":
-		if err := proc.cmd.Process.Kill(); err != nil {
+		// Use process group kill to ensure child processes
+		// (e.g. from shell pipelines) are also killed.
+		if err := signalProcess(proc.cmd.Process, syscall.SIGKILL); err != nil {
 			return xerrors.Errorf("kill process: %w", err)
 		}
 	case "terminate":
-		//nolint:revive // syscall.SIGTERM is portable enough
-		// for our supported platforms.
-		if err := proc.cmd.Process.Signal(syscall.SIGTERM); err != nil {
+		// Use process group signal to ensure child processes
+		// are also terminated.
+		if err := signalProcess(proc.cmd.Process, syscall.SIGTERM); err != nil {
 			return xerrors.Errorf("terminate process: %w", err)
 		}
 	default: