fix: Agent infrastructure critical issues (C3, C4, H1-H5)

[ULookup]

Summary

• C3: Add read_mutex_ to JsonRpcClient::read_message() to serialize concurrent reads from std::async tasks sharing the same stdout pipe
• H4: Add 30s timeout wrapper in send_request() to prevent indefinite blocking on hung child process
• C4: Change AgentLoop::run()/resume() return type from std::future<AgentResponse> to AgentResponse (all callers immediately .get() — async added no benefit but created dangling reference risk)
• H1: Delete AgentLoop move constructor/assignment (the object holds this in async paths)
• H2: Serialize actual tool call data and pipeline stats in checkpoint instead of hardcoded empty strings; track last_compaction_text_ for recovery
• H3: Replace [this] capture in init_runtime with explicit SubRunDeps struct (matching PipelineManager::Dependencies DI pattern)
• H5: Default-initialize PlanInput::on_escalate to []{} to prevent std::bad_function_call

Files Changed

• libs/core/include/merak/json_rpc_client.hpp — C3: add read_mutex_
• libs/core/src/json_rpc_client.cpp — C3: lock read_message + H4: 30s timeout
• libs/loop/include/merak/agent_loop.hpp — C4: sync return + H1: delete move + H2: last_compaction_text_
• libs/loop/src/agent_loop.cpp — C4: sync bodies + H2: checkpoint serialization + compaction tracking
• libs/app/src/application.cpp — C4: remove .get() + H3: SubRunDeps DI
• libs/loop/src/sub_agent_runner.cpp — C4: remove .get()
• libs/app/include/merak/app/application.hpp — H3: SubRunDeps struct + static execute_sub_run
• libs/core/include/merak/pipeline_types.hpp — H5: default init

Test plan

• CI passes
• Verify JSON-RPC requests no longer race on stdout pipe
• Verify hung child process is detected within 30s
• Verify AgentLoop checkpoint serializes real data
• Verify no dangling references in sub-agent execution paths
• Verify on_escalate no longer throws std::bad_function_call