Trace: capture Pi subagent evidence from a pinned result format #3713

Problem

Pi support records only the selected root session. The compatibility matrix marks Pi subagents as No, so child conversations, tool activity, and usage are absent from CHAINLOOP_AI_CODING_SESSION evidence.

Scope

Support the commit-pinned result format of Pi's official subagent example extension. Map identifiable child messages and usage into deterministic subagent evidence and raw-session streams.

Acceptance criteria

• Pin the supported official Pi subagent extension commit and supporting files.
• Parse single, parallel, and chained child results with deterministic child IDs and raw streams.
• Preserve child conversation content and tool results, not only aggregate token counts.
• Count child input, output, cache, and cost exactly once under the normalized accounting contract from Trace: define cache accounting and prevent token overcounting across providers #3706.
• Handle failed children and malformed optional result details without blocking push.
• Exclude abandoned root branches and unrelated extension records.
• Add provider/parser/action regressions and a live Pi E2E scenario.
• Update the Pi compatibility cell from measured results, using Partial if only the pinned official format is supported.

Non-goals

• Inferring subagents from parentSession alone.
• Supporting arbitrary third-party Pi extensions.
• Changing Pi's root session-tree selection contract.

Dependencies

• Trace: define cache accounting and prevent token overcounting across providers #3706 must define Pi cache and total-token semantics before child usage is aggregated.
• Baseline Pi support shipped in feat(trace): add Pi provider support #3704.