fix(pi): locate custom session directories exactly

[drewstone]

What changed

• read Pi session identity and working directory from the canonical first JSONL record
• support flat custom session directories used by PI_CODING_AGENT_SESSION_DIR
• reject unknown working directories when filtering
• align Traces 0.11.4 with Eval 0.142.2, Runtime 0.123.0, and Sandbox 0.17.2

Proof

• 587/587 tests passed
• typecheck, build, source-text check, and packed-package check passed
• deliberate fail-open mutation made the missing-working-directory test fail; restoring the condition returned 4/4 focused cases
• Pi + DeepSeek V4 Flash reviewed exact commit ab4af20 and approved
• clean merge tree against origin/main