agent-loop: new crate — LLM loop kernel + Model clients (phase 0)

Extract the LLM agent loop into a standalone workspace crate with zero
deps on skald-core/core-api (blueprint project-loop.md, D13-D15):

- kernel: round loop, model fallback with rebuild, parallel tool fan-out
  (ordered id alloc / bounded concurrent exec / ordered record), streaming
  deltas drained before outcomes, sticky cancellation
- models: OpenAiModel/AnthropicModel/OllamaModel/LmStudioModel ported from
  llm-client onto the Model trait; ModelError carries the HTTP status;
  is_retriable default = the 401/403/404/422 rule
- DTL as crate protocol (ToolRendering Inline/DeferredToolReference/
  SystemToolBlock; Anthropic conversions + Kimi system+tools passthrough),
  host catalog behind ActivationSource/ToolActivator
- HistoryStore durability contract + InMemoryStore; LinearAssembler with
  well-formed projection (incl. DTL injection, summary, crash survivors)
- LoopManager singleton (broadcast bus + live registry), one live loop
  per conversation, orphan-marking on start_turn
- 32 tests green (kernel §13 suite, assembler DTL, SSE/Anthropic ports),
  clippy clean

Cargo.lock

@@ -43,6 +43,22 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "agent-loop"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "async-trait",
+ "futures",
+ "futures-util",
+ "reqwest 0.13.4",
+ "serde",
+ "serde_json",
+ "tokio",
+ "tokio-util",
+ "tracing",
+]
+
 [[package]]
 name = "aho-corasick"
 version = "1.1.4"