fix(host_agent): deploy/provision 500 — double-begin on session

fetch_scope_targets() runs a SELECT that autobegins the session transaction
(SQLAlchemy 2.0), so the following `async with db.begin()` raised
"A transaction is already begun on this Session" → 500 on both
/plugins/host_agent/deploy and /provision.

Mint registrations directly into the autobegun transaction, build the
inventory while the ORM objects are still live, then await db.commit().
Escaped CI because unit tests use a mock db_sessionmaker; no integration
test exercises these routes (follow-up noted in Scribe issue #884).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/host_agent/routes.py

@@ -731,17 +731,19 @@ async def deploy_via_ansible():
         targets = await fetch_scope_targets(db, scope)
         if not targets:
             return _error(400, "no_targets", "No Ansible targets in that scope")
+        # The read above autobegins the session transaction (SQLAlchemy 2.0), so
+        # mint into it directly and commit — a nested db.begin() would double-begin.
         tokens: dict[str, str] = {}
-        async with db.begin():
-            for t in targets:
-                host = await _ensure_host_for_target(db, t)
-                tokens[t.name] = await _mint_registration_token(db, host)
-        inv = generate_inventory(targets)
+        for t in targets:
+            host = await _ensure_host_for_target(db, t)
+            tokens[t.name] = await _mint_registration_token(db, host)
+        inv = generate_inventory(targets)  # built while ORM objects are still live
         for name, tok in tokens.items():
             hv = inv["_meta"]["hostvars"].setdefault(name, {})
             hv["steward_url"] = url
             hv["steward_token"] = tok
         inventory_content = _json.dumps(inv)
+        await db.commit()
 
     actor_role = UserRole(session.get("user_role", "viewer"))
     run, _source, err = await invoke_capability(
@@ -805,12 +807,13 @@ async def provision_via_ansible():
         targets = await fetch_scope_targets(db, scope)
         if not targets:
             return _error(400, "no_targets", "No Ansible targets in that scope")
+        # The read above autobegins the session transaction (SQLAlchemy 2.0), so
+        # mint into it directly and commit — a nested db.begin() would double-begin.
         tokens: dict[str, str] = {}
-        async with db.begin():
-            for t in targets:
-                host = await _ensure_host_for_target(db, t)
-                tokens[t.name] = await _mint_registration_token(db, host)
-        inv = generate_inventory(targets)
+        for t in targets:
+            host = await _ensure_host_for_target(db, t)
+            tokens[t.name] = await _mint_registration_token(db, host)
+        inv = generate_inventory(targets)  # built while ORM objects are still live
         for name, tok in tokens.items():
             hv = inv["_meta"]["hostvars"].setdefault(name, {})
             hv["steward_url"] = url
@@ -821,6 +824,7 @@ async def provision_via_ansible():
             hv["steward_pubkey"] = pubkey
             hv["steward_user"] = steward_user
         inventory_content = _json.dumps(inv)
+        await db.commit()
 
     actor_role = UserRole(session.get("user_role", "viewer"))
     run, _source, err = await invoke_capability(