feat(agents): enable Copilot SDK infinite sessions with auto-compaction

[tag-assistant]

Summary

Enables the SDK's infinite sessions feature so long-running copilot sessions auto-compact context instead of hitting token limits.

Changes

• copilot-sdk.ts — accepts infiniteSessions config, passes to SessionConfig, returns workspacePath
• copilot-runner.ts — enables infinite sessions by default, reads custom thresholds from CLI backend config
• pi-embedded-runner/types.ts — adds workspacePath to result meta
• Tests: 4 SDK tests + 2 runner tests

Default Config

• Background compaction at 80% context utilization
• Hard block at 95% (waits for compaction to finish)
• Enabled by default for all copilot-cli runs

Part of #4 (Copilot SDK Deep Integration)