refactor(backend): base owns the gateway-attach flow, fail hard (0081 review)

Addresses review on #519 (@didericis 6143 + the fail-hard direction over the
codex skip).

Base owns the flow (6143 / ADR 0006). `attach_bottled_agents_to_gateway()` is
now a concrete method on `BottleBackend` that delegates to
`gateway_attach.reconcile_running_bottles`; backends override only three
primitives — `_gateway_attach_resources()`, `_running_bottles()`,
`_attach_bottle_to_gateway()`. The shared control flow + error policy live in
one place so a backend can't drift onto a bespoke loop or a silent skip. Keeps
the reconcile flow + `GatewayAttachResources` out of `backend/base.py` (the
size guardrail) in a dedicated `backend/gateway_attach.py`. New ADR 0006 records
the "shared behaviour in the base backend, subclasses override primitives"
theme.

Fail hard, never skip (the maintainer's direction over the codex review's
skip). Any attach failure now aborts bring-up instead of being logged and
skipped: a bottle that silently can't reach the fresh gateway (its egress just
starts failing TLS) is worse than a loud failure. Every bottle is attempted and
the failures are raised together (aggregate `InfraLaunchError`) so one bring-up
surfaces the full blast radius. Resource gathering (the CA fetch) also fails
hard. PRD 0081 goal + design updated to match (reversed from the earlier
"tolerate per-bottle failures" draft).

Bumps the base.py guardrail cap 580->600 for the new contract surface (the
delegator + 3 abstract primitives); the flow itself lives in gateway_attach.py.

Refs #516, #519.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_architecture_guardrails.py

@@ -83,8 +83,13 @@ class TestRuntimeModuleSizes(unittest.TestCase):
         self.assertEqual([], violations)
 
     def test_backend_contract_does_not_absorb_preparation_logic(self) -> None:
+        # base.py holds the backend *contract*; implementation lives elsewhere.
+        # The cap is a tripwire against absorbing preparation/impl logic, not a
+        # ban on new contract surface — bumped 580->600 for the PRD 0081
+        # gateway-attach contract (delegator + 3 abstract primitives; the flow
+        # itself lives in backend/gateway_attach.py, not here).
         caps = {
-            ROOT / "bot_bottle" / "backend" / "base.py": 580,
+            ROOT / "bot_bottle" / "backend" / "base.py": 600,
             ROOT / "bot_bottle" / "backend" / "preparation.py": 160,
         }
         oversized = [