docs: clarify agent output and trajectory concepts

CONTEXT.md

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+# Code Agent Runtime
+
+This glossary defines the runtime artifacts produced and maintained by a code agent so that public output, diagnostics, evaluation data, and resumable state remain distinct.
+
+## Language
+
+**Agent Run**:
+A bounded execution that starts with an instruction and ends when the agent stops autonomous work.
+_Avoid_: Session, turn
+
+**Run Output**:
+The public result or event protocol that one agent run emits to its caller.
+_Avoid_: Trace, transcript
+
+**Run Result Object**:
+A single public object that summarizes how one Agent Run ended, including its final result and optional usage metadata.
+_Avoid_: Trajectory, session export
+
+**Public Event Stream**:
+A caller-facing Run Output that exposes a stable, selected sequence of Agent Run events as they occur.
+_Avoid_: Internal event bus, execution trace
+
+**Partial Content**:
+An unfinished representation of a message, reasoning block, or tool input that is exposed before the logical content is complete.
+_Avoid_: Final message, completed event
+
+**Content Delta**:
+The incremental fragment or change relative to content already emitted for the same logical item.
+_Avoid_: Partial snapshot, final message
+
+**Terminal Event**:
+The final Public Event Stream record that explicitly states how an Agent Run ended.
+_Avoid_: EOF, last message
+
+**Execution Trace**:
+A diagnostic record of runtime activity retained to explain failures, timing, and internal behavior.
+_Avoid_: Run output, trajectory
+
+**Trajectory**:
+A task-scoped record of observations, actions, results, and outcome prepared for evaluation or offline analysis.
+_Avoid_: Execution trace, session, transcript
+
+**Session**:
+Durable agent state that can span multiple runs and supports continuing or branching prior work.
+_Avoid_: Agent run, trajectory, transcript