feat(sandbox): expose Workdir field in SandboxConfig (#853)

The internal sandbox.Config already supports a custom container Workdir
that doubles as the workspace mount target, but config.SandboxConfig
(the JSONB-backed per-agent config) did not expose it. Without this,
agents must use the hardcoded /workspace mount point even when their
scripts expect a different in-container path.

Adds an optional "workdir" JSON field that maps through to
sandbox.Config.Workdir. Backward compatible — existing agents without
workdir set fall back to the /workspace default.

Co-authored-by: tuannt23065 <tuannt23065@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -426,6 +426,7 @@ type SandboxConfig struct {
 	User           string `json:"user,omitempty"`             // container user (e.g. "1000:1000", "nobody")
 	TmpfsSizeMB    int    `json:"tmpfs_size_mb,omitempty"`    // default tmpfs size in MB (0 = Docker default)
 	MaxOutputBytes int    `json:"max_output_bytes,omitempty"` // limit exec output capture (default 1MB)
+	Workdir        string `json:"workdir,omitempty"`          // container workdir + workspace mount target (default "/workspace")
 
 	// Pruning (matching TS SandboxPruneSettings)
 	IdleHours        int `json:"idle_hours,omitempty"`         // prune containers idle > N hours (default 24)
@@ -499,6 +500,9 @@ func (sc *SandboxConfig) ToSandboxConfig() sandbox.Config {
 	if sc.MaxOutputBytes > 0 {
 		cfg.MaxOutputBytes = sc.MaxOutputBytes
 	}
+	if sc.Workdir != "" {
+		cfg.Workdir = sc.Workdir
+	}
 
 	// Pruning
 	if sc.IdleHours > 0 {