feat(F): F-CTX context_builder, F-STR agent_stream, F-DEF agent definition fields, F-SKILL skill_registry, F-TOOL agent_tools

- F-CTX: app/ai/context_builder.py (282 lines) — build_agent_context() + ReActSystemPromptBuilder
- F-STR: app/ai/agent_stream.py (155 lines) — stream_react_loop() with SSE events (step, status, done, error)
- F-DEF: AgentDefinition fields added (temperature, max_tokens, max_steps, trace_mode, skill_ids, trigger_config, ai_use_case_metadata) + migration 0122
- F-SKILL: app/ai/skill_registry.py (82 lines) — SkillDefinition + SkillRegistry singleton
- F-TOOL: app/ai/agent_tools.py (117 lines) — get_agent_tools() with permission intersection
- Skill CRUD routes: app/plugins/builtins/automation/skill_routes.py
- Tests: test_skill_registry.py (97 lines), test_agent_tools.py (219 lines)
- All Python compile checks pass, tests require PostgreSQL (infra issue, not code bug)

app/plugins/builtins/automation/plugin.py

@@ -50,6 +50,11 @@ class AutomationPlugin(BasePlugin):
                 module="app.plugins.builtins.automation.agent_routes",
                 router_attr="router",
             ),
+            PluginRouteDef(
+                path="/api/v1/skills",
+                module="app.plugins.builtins.automation.skill_routes",
+                router_attr="router",
+            ),
         ],
         events=[
             "contact.created",
@@ -57,7 +62,7 @@ class AutomationPlugin(BasePlugin):
             "mail.received",
             "workflow.timeout",
         ],
-        migrations=["0001_initial.sql", "0002_agent_subtasks.sql"],
+        migrations=["0001_initial.sql", "0002_agent_subtasks.sql", "0003_skill_definitions.sql"],
         permissions=[
             "automation:read",
             "automation:write",