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>

oubliette.toml.example

@@ -50,6 +50,12 @@ hostname = "ubuntu-server"
 # banner = "Welcome to Ubuntu 22.04.3 LTS (GNU/Linux 5.15.0-89-generic x86_64)\r\n\r\n"
 # fake_user = ""  # override username in prompt; empty = use authenticated user
 
+# Map usernames to specific shells (regardless of how auth succeeded).
+# Credential-specific shell overrides take priority over username routes.
+# [shell.username_routes]
+# postgres = "psql"
+# admin = "bash"
+
 # Per-shell configuration (optional).
 # [shell.banking]
 # bank_name = "SECUREBANK"
@@ -65,6 +71,10 @@ hostname = "ubuntu-server"
 # ios_version = "15.0(2)SE11"
 # enable_password = ""  # empty = accept after 1 failed attempt
 
+# [shell.psql]
+# db_name = "postgres"
+# pg_version = "15.4"
+
 # [detection]
 # enabled = true
 # threshold = 0.6          # 0.0–1.0, sessions above this trigger notifications