Trace: record Pi skill loads in session evidence #3714

Problem

The compatibility matrix marks Pi skills tracking as No. Pi can load project skills, but its provider does not implement trace.SkillTracker, so evidence contains no skill package or use records.

Scope

Implement Pi skill tracking through the shared SkillTracker contract. Detect completed filesystem skill loads from the live Pi transcript at hook time, snapshot the skill package once, and retain push-time fallback behavior for missed loads.

Acceptance criteria

• Implement NewSkillLoads, SessionSkills, and SkillRoots for Pi with compile-time interface coverage.
• Detect no-use, repeated-use, and multiple completed skill loads in first-use order.
• Classify project, user, organization, and plugin roots consistently with shared packaging.
• Snapshot skill contents at use time so later edits do not silently replace the recorded version.
• Preserve the existing warning when a snapshot can only be made at push.
• Ignore incomplete or failed load attempts.
• Add provider/action regressions and a live Pi skill scenario that inspects stored evidence.
• Update the compatibility matrix from measured results.

Non-goals

• Redesigning the shared skill evidence schema.
• Tracking skills that Pi never exposes through filesystem or transcript records.

References

• Shared skills design: feat(trace): record the skills that an agent used in a session #3540.
• Baseline Pi support: feat(trace): add Pi provider support #3704.