Title: Add structured logging and session telemetry (opt-in)

[Mkalbani]

Description

There is currently no way to diagnose player-reported issues (e.g. "hint gave wrong answer") without reproducing locally. Add structured, opt-in logging that records session events for debugging, with no telemetry sent anywhere by default.

Acceptance Criteria

• tracing crate integrated with structured spans for session start/end, answer submission, and hint reveal
• --debug-log <path> CLI flag writes JSON-lines logs to a file; logging is off by default
• No logs are transmitted over the network under any configuration
• Log format documented in docs/logging.md
• Sensitive data (if any) is excluded or redacted from logs

[grantfox-oss]

🦊 GrantFox — @jaynomyaro has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #32)
2. Your PR will be reviewed by the ThinknetCollective maintainers

Good luck! Track your progress on GrantFox.