Fix session recording path linkage

[pranc1ngpegasus]

Summary

• Capture the recording start timestamp and directory name once when recording starts
• Pass them through the session runner so the persisted session row points at the actual recording directory
• Add a regression test for session path persistence

Verification

• cargo fmt --check
• cargo test -p wisp-storage -p wisp-core -p wisp-mcp

Note: wisp-desktop build/test is blocked in this environment because gpui requires Xcode's metal utility, which is not installed.