feat: add psql shell and username-to-shell routing

Add a PostgreSQL psql interactive terminal shell with backslash
meta-commands, SQL statement handling with multi-line buffering, and
canned responses for common queries. Add username-based shell routing
via [shell.username_routes] config (second priority after credential-
specific shell, before random selection). Bump version to 0.13.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -28,10 +28,11 @@ type WebConfig struct {
 }
 
 type ShellConfig struct {
-	Hostname string                    `toml:"hostname"`
-	Banner   string                    `toml:"banner"`
-	FakeUser string                    `toml:"fake_user"`
-	Shells   map[string]map[string]any `toml:"-"` // per-shell config extracted manually
+	Hostname       string                    `toml:"hostname"`
+	Banner         string                    `toml:"banner"`
+	FakeUser       string                    `toml:"fake_user"`
+	UsernameRoutes map[string]string         `toml:"username_routes"`
+	Shells         map[string]map[string]any `toml:"-"` // per-shell config extracted manually
 }
 
 type StorageConfig struct {
@@ -165,9 +166,10 @@ func applyDefaults(cfg *Config) {
 
 // knownShellKeys are top-level keys in [shell] that are not per-shell sub-tables.
 var knownShellKeys = map[string]bool{
-	"hostname":  true,
-	"banner":    true,
-	"fake_user": true,
+	"hostname":        true,
+	"banner":          true,
+	"fake_user":       true,
+	"username_routes": true,
 }
 
 // extractShellTables pulls per-shell config sub-tables from the raw [shell] section.