feat(persistence): PostgreSQL assistant store, chat persist fixes, gateway scripts

- Add SQLAlchemy Core repos, pg adapter/compat, assistant_store factory, Alembic bootstrap and migration/cutover scripts.

- Harden chat_message writes (NUL scrub for PG), turn_uuid on attempt failure, WS turn_runner fallbacks and gateway executed_turn_uuid init.

- start_gateway: log paths, PS7 stderr handling via cmd, background stdout/stderr redirect; runtime assistant_runtime_log_dir export.

- Ops: clear_all_chat_sessions with PG-only --postgresql and env-gated wipe; clear_postgres_chat_sessions.ps1.

- Tests: SA repos, pg compat, persist fallback, smoke env isolation; CI and docs touch-ups.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/clear_postgres_chat_sessions.ps1

@@ -0,0 +1,4 @@
+$ErrorActionPreference = "Stop"
+$real = Join-Path $PSScriptRoot "..\runtime\operations\scripts\clear_postgres_chat_sessions.ps1"
+if (-not (Test-Path $real)) { throw "Forward target not found: $real" }
+& $real @args

scripts/start_gateway.ps1

@@ -3,7 +3,8 @@ param(
     [int]$Port = 8787,
     [switch]$SkipInstall = $false,
     [switch]$Background = $false,
-    [bool]$WithWikiWorker = $true
+    [bool]$WithWikiWorker = $true,
+    [switch]$NoLogMirror = $false
 )
 
 $ErrorActionPreference = "Stop"
@@ -13,5 +14,5 @@ if (-not (Test-Path $real)) {
     throw "Forward script target not found: $real"
 }
 
-& $real -BindHost $BindHost -Port $Port -SkipInstall:$SkipInstall -Background:$Background -WithWikiWorker:$WithWikiWorker
+& $real -BindHost $BindHost -Port $Port -SkipInstall:$SkipInstall -Background:$Background -WithWikiWorker:$WithWikiWorker -NoLogMirror:$NoLogMirror