Trace: record Pi root context and compactions #3721

Problem

Pi Trace evidence has session-wide token totals but not root-session context-window occupancy or native compaction events. The broader multi-provider request is #3637.

Scope

Pi root selected branch only. Add the minimum machine-readable context size, peak/final occupied tokens, and compaction count that native Pi 1.3.0 records can substantiate.

Acceptance criteria

• Identify Pi's authoritative window size and per-turn occupancy from pinned transcript/model metadata; do not infer a denominator from a model name when Pi does not provide one.
• Record root-session peak/final context and native compaction count with explicitly versioned evidence fields; percentages appear only with a verified window size.
• Count selected-branch and resumed history exactly once; exclude abandoned branches and child sessions.
• Cover empty, malformed, missing-window, and multi-compaction transcripts in deterministic regressions.
• Validate the evidence schema/consumer compatibility and a live Pi session before updating only the measured Pi matrix cell.

Non-goals

• Other providers or child-session context metrics.
• The separate git_context.commit_start observation in Trace: record context window usage and compactions in AI coding session evidence #3637.
• Estimating context size from cumulative session token totals.

Parent: #3637. Baseline Pi provider: #3704.