Session budgets and redacted exports in eve

Four small files under agent/ do the work, each with one job: limits.sessionUsage in agent.ts stops a runaway turn; instrumentation/otel.ts sets the most content any trace destination can get; instrumentation/tool-timing.ts records durations without content; instrumentation/file-export.ts is a destination with its own, stricter export policy. The decision that matters most is that redaction happens in layers: a client's name typed into one tool argument also appears in the model's input messages, its tool calls and any tool result that echoes a document, so scrubbing one field would leak in three other places. The destination drops whole content directions first, then scrubs the one content attribute it keeps.