feat: add Pi memory integration

[bluewatercg]

Summary

• add Pi session discovery and active-branch history import
• install a native Pi extension for L1 recall, settled-turn capture, and /memmy-resume
• make live capture fail-open while preserving failed/aborted status, redacting secrets, and preventing live/history duplicates
• expose Pi across backend registration, auto-injection, desktop source UI, and bilingual docs

Validation

• npm run lint -w @memmy/backend
• npm run typecheck -w @memmy/backend
• 45 affected backend tests passed (21 core + 24 route)
• 22 frontend source-page tests passed
• npm run version:check
• git diff --cached --check

Known baseline

• full frontend typecheck still reports the existing 5 errors in app-frame.test.tsx and project-target-picker.interaction.test.tsx; this PR adds no new typecheck errors

Scope

• L1 only; no L2/L3 changes
• version, release notes, packaging, and clean-upgrade validation are intentionally deferred until after merge