M1 core: tool trait, permission service, config, Provider trait, engine loop

harness-core now has everything the headless agent loop needs:
- Tool trait/ToolCtx/ToolRegistry + 30k-char head+tail output truncation
- PermissionService: async ask over a oneshot + AppEvent::PermissionAsked,
  Once/Always/Reject replies, an auto-approve stub for tests/headless runs
- Config: JSONC loading, bundled/global/project-chain/env precedence,
  {env:VAR} and {file:path} interpolation
- llm.rs: LlmEvent/LlmRequest/Provider trait, wire message/content types
- engine/: outer loop (run_session), inner stream processor (persists
  parts/messages as events arrive, executes tool calls inline), retry
  policy (retries only the pre-first-event window), doom-loop guard,
  system prompt assembly

Verified end-to-end against a scripted MockProvider: text -> tool call
(read) -> final text, with messages/parts persisted in the right shape,
plus a provider-error-before-any-event case surfacing as Errored (no
partial message left behind). 49 tests passing, clippy clean.

crates/harness-core/src/engine/mod.rs

@@ -0,0 +1,10 @@
+pub mod doomloop;
+pub mod processor;
+pub mod retry;
+#[path = "loop.rs"]
+pub mod session_loop;
+pub mod system;
+
+pub use doomloop::DoomLoopGuard;
+pub use processor::{process_step, StepContext, StepError, StepOutcome, StepResult};
+pub use session_loop::{run_session, RunConfig};