Remove the phased agent code

[yichao-liang]

Summary

Stacked on #189 (base empiric-configs); review and merge after it.

The EMPIRIC agent and its six baselines all run under the continual protocol, where each arm plays the levels of one environment through ContinualPlayMixin.play_level.
This deletes the phased experiment path (learn sessions between online learning cycles, solve sessions per test task) and the code only that path reached: 70 files, about 12.8k lines.

Deleted:

• AgentModelBasedApproach, the agent explorers and their base class, and the subgoal_annotations execution monitor;
• SamplerLearningMixin and the sampler synthesis tools;
• the NL world model and GNN dynamics baselines;
• the Docker sandbox backend and its agent runner;
• the learn-notes prompt templates and their goldens;
• in AgentSimLearningApproach: the methods only the phased path called, the oracle-simulator branches, and the per-cycle fit diagnostics with their fields;
• the settings flags that lost their last reader.

Kept:

• AgentModelFreeApproach keeps the option model, tool-context sync and checkpointing that the continual arms inherit. Its _solve now raises, since no arm solves tasks outside play_level.
• ExoPredicator-era code, the arm classes outside the benchmark, the env variants and the real-robot stack. No deleted file exists at the ExoPredicator tag or at the upstream fork point.

Sketch refinement now draws a step's params from its ground sampler, or uniformly; the learned parameterized samplers were phased-only.

empiric/common.yaml and envs.yaml stop setting the phased pipeline's flags (make_*_videos, num_online_learning_cycles, max_num_steps_interaction_request, the online early-stopping flags, online_nsrt_learning_requests_per_cycle).
Under the continual protocol run_pipeline returns right after run_continual, so none of them was read.

Tests of deleted code are deleted.
test_execution_monitoring.py returns to its upstream version, and the interaction-request bookkeeping test now exercises the online process learner, the remaining implementer of that hook.

Test plan

• yapf, isort 5.10.1, docformatter, mypy and pylint on this commit (sbatch replay of CI's static jobs).
• The 8 CI shards in the CI container.

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