observability: per-turn telemetry for harness sessions

[OmGuptaIND]

Why

A "loading then stops" report on a codex turn exposed two diagnostic gaps:

• The 107 s gap between the last shell tool call and the model interruption looked identical in logs to a real stall — there was no signal that reasoning deltas were flowing.
• A subsequent turn ran python3 -m http.server over real index.html / styles.css files, but no apply_patch operation showed up anywhere in the log. File writes were invisible.

Both gaps are generic to every harness we run, not codex-specific.

What

Adds packages/agent-core/src/harness/turn-telemetry.ts — a small provider-agnostic helper used by both CodexHarnessSession and HarnessSession (claude-code). It owns:

• Per-turn counters derived from SessionEvent types: thinking → reasoningChars, text → messageChars, tool_call → toolCalls, artifact (artifactType: 'file') → fileChanges.
• Idle watchdog (5 s tick, 30 s threshold, debounced): warns with running counters when the event stream goes silent — distinguishes "model is reasoning" from "CLI/network is stuck".
• turn started / turn completed log lines, with the summary attributing wall-clock time to what the model actually did.

Codex keeps its protocol-specific lifecycle logs (codex reasoning started/completed with per-item char counts, codex file_change with paths/kinds) inline since they need the raw item/* shapes the generic SessionEvent stream doesn't expose.

What this changes about debugging

Replaying the original incident, the turn completed log now answers "what did this turn do?" at a glance:

• Turn 1 (interrupted): reasoningChars: ~5k, messageChars: 1261, fileChanges: 0, toolCalls: 5 — planned, didn't build.
• Turn 2 (the one that worked): … fileChanges: 4, toolCalls: 7 — built and served the site.
• Turn 3 (the failed follow-up): messageChars: 542, fileChanges: 0, toolCalls: 0 — talked, did nothing.

Previously all three looked similar in logs.

Test plan

• tsc --noEmit passes (packages/agent-core)
• biome check passes on changed files
• Send a multi-turn message in codex harness; confirm turn started / codex reasoning started / codex file_change / turn completed appear with sensible counters
• Send a message in claude-code harness; confirm turn started / turn completed appear
• Run a long-thinking turn and confirm turn idle warns ~30 s in with non-zero reasoningChars
• Cancel a turn mid-stream and confirm only one turn completed line lands (no duplicate from the finally-block safety net)

🤖 Generated with Claude Code