feat: round-11 abstractions — RuntimeConfig auto thread/core, ShutdownGuard RAII

.hermes/plans/mytheclipse-round11-spec.md

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+# Implementation Spec: Round 11 — COMPLETE
+
+## Goal
+Auto thread/core allocation + race hardening (RAII shutdown).
+
+## New Features
+
+### 1. RuntimeConfig (mytheclipse-core, lifecycle)
+File: `crates/mytheclipse/src/runtime_auto.rs`
+- `RuntimeConfig::auto()` / `from_cores(n)` / `compact()` infer worker_threads,
+  max_blocking_threads, compute_threads, io_threads from host CPU topology
+  (std::thread::available_parallelism)
+- `available_parallelism()` helper
+- `build_rayon_pool(cfg)` gated on `compute` feature
+- 3 tests
+
+### 2. ShutdownGuard (mytheclipse-core, lifecycle)
+File: `crates/mytheclipse/src/shutdown_guard.rs`
+- RAII guard — runs completion callback exactly once on drop (panic-safe via
+  Mutex<Option<Box<FnOnce>>>), prevents double-shutdown race
+- `new(cb)` + `finish()` (fire now + disarm)
+- 3 tests (fires on drop, finish once, panic path)
+
+## Files
+- new: core/src/runtime_auto.rs, core/src/shutdown_guard.rs
+- core/lib.rs: +module+export for both
+
+Build: exit 0. Tests: 0 FAILED. Clippy: 0 new warnings.