perf(core): offload score_blocks_mig O(K²) loop to spawn_blocking

[bug-ops]

Summary

• Wraps the O(K²) pairwise MIG scoring loop in run_focus_auto_consolidation with tokio::task::spawn_blocking to prevent blocking the async executor during long-session consolidation passes.
• Messages and task goal are cloned before entering the blocking closure; a fresh TokenCounter is constructed inside it.
• No change to score_blocks_mig signature or the sync call site in prune_tool_outputs_mig (which is already a sync fn).

Closes #3386

Test plan

• cargo build -p zeph-core — compiles cleanly
• cargo +nightly fmt --check — no formatting issues
• cargo clippy -p zeph-core -- -D warnings — zero warnings
• cargo nextest run --workspace --lib --bins — 8475 tests pass, 0 failures