feat(core): agent orchestration & context compression optimization

[bobleer]

Summary

Multi-layered improvements to agent stability and efficiency, referencing Claude Code's architecture patterns for context compression and tool orchestration.

Tool Execution Fault Tolerance & Mixed Scheduling

• execute_tools fault tolerance: Convert execute_tools ? to match in round_executor — on pipeline-level failure, generate is_error: true error results for every tool_call instead of aborting the entire round
• get_task None fallback: When get_task returns None in tool_pipeline, use task_id as fallback to generate error results, ensuring tool_call/tool_result count always matches
• Partitioned mixed scheduling: Replace all-parallel/all-serial tool execution with continuous concurrency-safety partitioning — safe tools run in parallel batches, non-safe tools run serially, batches execute in order to preserve write-after-read dependencies

Multi-layered Context Compression

• L0 Microcompact: Lightweight pre-compression that runs before full compression, replacing old compactable tool results (Read/Bash/Grep/Glob and 8 others) with placeholders while keeping the 8 most recent intact (includes 4 unit tests)
• L2 Emergency truncation: After all compression layers, if tokens still exceed context_window, drop oldest API rounds from context head until tokens fit
• Compression circuit breaker: Skip full compression after 3 consecutive failures (microcompact + emergency truncation always run as fallback)
• Unified context_window: Both execution_engine and coordinator manual compaction now use min(model_capability, session_config)

Resource Cleanup

• Synchronously delete SessionContextStore in-memory data when idle sessions are evicted, preventing memory leaks

Changes

File	Change
round_executor.rs	execute_tools ? → match fault tolerance
tool_pipeline.rs	get_task fallback + partitioned mixed scheduling
microcompact.rs	New Microcompact layer (with tests)
execution_engine.rs	Microcompact integration + emergency truncation + circuit breaker + context_window unification
coordinator.rs	Manual compaction context_window unification
session_manager.rs	Eviction cleanup for context_store
message.rs	Make estimate_tokens pub
compression/mod.rs	Export microcompact module

Test plan

• cargo check — zero errors, zero warnings
• cargo test -p bitfun-core — all 202 tests pass (including 4 new microcompact tests)
• Manual test: long conversation scenario to verify microcompact clearing effect
• Manual test: mixed tool calls (concurrent + serial) to verify partitioned scheduling
• Manual test: compression failure scenario to verify circuit breaker + emergency truncation fallback