feat(automation): add empty Capability / Action / Trigger registries

Three registries under app/automations/registries/, each as its own
folder with the same SRP-per-file split (types.py for the dataclass,
store.py for the in-memory dict + register/get/all functions). All
three start empty; concrete entries land when the user signs off on
which capabilities / actions / triggers to include (step 2).

Capability (locked at v1-minimum five fields — see commit 2):
  - id, description, input_schema, output_schema, handler
  - CapabilityHandler = Callable[[dict[str, Any]], Awaitable[Any]]
  - Frozen, slotted dataclass (immutable post-registration).

ActionDefinition (v1-trim of design plan §4):
  - type, name, description, config_schema, handler
  - Defers output_contract (handled per-step by agent_task's
    config.output_schema), uses_capabilities (no static analysis
    needed until >1 action ships), and produces_artifacts (deferred
    alongside the artifact pipeline).

TriggerDefinition (declarative, no handler):
  - type, description, config_schema, payload_schema
  - No handler field — firing is a single dispatcher's
    responsibility, not a per-trigger one.

store.py contract for all three:
  - register_*: idempotent at process startup, raises on duplicate
  - get_*: returns None on miss
  - all_*: returns a defensive copy of the registry dict

Verified by an inline smoke test (10 checks): empty initial state,
registration and lookup work, duplicates raise, frozen dataclasses
reject mutation, snapshots are copies, handlers are awaitable.

Isolation invariant audit: grep across the full app/automations/
tree shows only three app.* imports, all of them
``from app.db import BaseModel, TimestampMixin`` in the model files.
No imports from app.agents.*, app.services.*, app.tasks.*,
app.routes.*, or any other business-logic module.

surfsense_backend/app/automations/registries/capabilities/__init__.py

@@ -2,4 +2,13 @@
 
 from __future__ import annotations
 
-__all__: list[str] = []
+from .store import all_capabilities, get_capability, register_capability
+from .types import Capability, CapabilityHandler
+
+__all__ = [
+    "Capability",
+    "CapabilityHandler",
+    "all_capabilities",
+    "get_capability",
+    "register_capability",
+]

surfsense_backend/app/automations/registries/capabilities/store.py

@@ -0,0 +1,40 @@
+"""Capability registry: in-memory dict + ``register_capability`` API."""
+
+from __future__ import annotations
+
+from .types import Capability
+
+_REGISTRY: dict[str, Capability] = {}
+
+
+def register_capability(capability: Capability) -> None:
+    """Add a capability to the in-memory registry.
+
+    Raises ``ValueError`` on duplicate ``id`` — registration is
+    idempotent only at the module level (a module's
+    ``register_capability`` call runs once per process), so a
+    duplicate is always a bug.
+    """
+
+    if capability.id in _REGISTRY:
+        raise ValueError(
+            f"Capability already registered: {capability.id!r}"
+        )
+    _REGISTRY[capability.id] = capability
+
+
+def get_capability(capability_id: str) -> Capability | None:
+    """Look up one capability by id. Returns ``None`` on miss."""
+
+    return _REGISTRY.get(capability_id)
+
+
+def all_capabilities() -> dict[str, Capability]:
+    """Snapshot of the registry as a defensive copy.
+
+    Returned dict is safe to iterate while other code calls
+    ``register_capability`` (which v1 never does post-startup, but
+    the contract holds anyway).
+    """
+
+    return dict(_REGISTRY)

surfsense_backend/app/automations/registries/capabilities/types.py

@@ -0,0 +1,40 @@
+"""``Capability`` dataclass — the v1-minimum five-field shape."""
+
+from __future__ import annotations
+
+from collections.abc import Awaitable, Callable
+from dataclasses import dataclass
+from typing import Any
+
+CapabilityHandler = Callable[[dict[str, Any]], Awaitable[Any]]
+"""The signature every capability handler must satisfy.
+
+The handler is a closure that already holds whatever runtime context
+it needs (DB session, search-space scope, logger, etc.). The
+registry only passes through the caller's input dict — the same dict
+that was validated against ``input_schema``.
+"""
+
+
+@dataclass(frozen=True, slots=True)
+class Capability:
+    """The unit of "what SurfSense can do," consumed by every layer.
+
+    v1 keeps the dataclass to exactly five fields. Earlier drafts
+    considered ``name``, ``required_credentials``, ``side_effects``,
+    ``expected_duration_seconds``, and ``cost_estimate``; every one
+    of those has been removed until a concrete consumer feature
+    requires it (see ``automation-design-plan.md`` §3, decision v1).
+
+    The handler is a ready-to-call function. It does not receive a
+    context argument — context is bound at registration time by the
+    factory that builds the closure (so a capability returned to an
+    agent's tool list looks identical to one returned to an
+    automation's action runtime).
+    """
+
+    id: str
+    description: str
+    input_schema: dict[str, Any]
+    output_schema: dict[str, Any]
+    handler: CapabilityHandler