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/migrations/0003_skill_definitions.sql

@@ -0,0 +1,18 @@
+-- Skill Definitions for AI Agent Skills (Phase F-SKILL)
+
+CREATE TABLE IF NOT EXISTS automation_skill_definitions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    tenant_id UUID NOT NULL,
+    name VARCHAR(255) NOT NULL UNIQUE,
+    description TEXT NOT NULL,
+    instructions TEXT NOT NULL,
+    allowed_tool_ids JSONB NOT NULL DEFAULT '[]',
+    context_policy JSONB,
+    category VARCHAR(100) NOT NULL DEFAULT 'general',
+    is_active BOOLEAN NOT NULL DEFAULT TRUE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    deleted_at TIMESTAMPTZ
+);
+CREATE INDEX IF NOT EXISTS ix_skill_defs_tenant_active ON automation_skill_definitions (tenant_id, is_active);
+CREATE INDEX IF NOT EXISTS ix_skill_defs_tenant_category ON automation_skill_definitions (tenant_id, category);