feat: add structured logging

[assapir]

Problem

Golem currently uses println!/eprintln! for all output — user-facing messages, debug info, warnings, and errors are all mixed together with no log levels, timestamps, or filtering.

Proposed solution

Add structured logging via the tracing crate (or log + env_logger for simplicity).

What to log

Level	Examples
error	API failures, auth errors, tool execution failures
warn	Parse retries, deprecated features, fallback behavior
info	Model changes, login/logout, tool execution summaries
debug	Full API request/response bodies, ReAct loop iterations, memory operations
trace	Raw HTTP headers, token refresh flow, config reads

Configuration

• Default: warn level (quiet for normal use)
• --verbose / -v flag: info level
• --debug flag or GOLEM_LOG=debug env var: debug level
• /debug slash command to toggle at runtime (feat: add debug mode to show raw LLM request/response data #38)

Implementation

• Add tracing + tracing-subscriber dependencies
• Replace eprintln!("warning: ...") with tracing::warn!()
• Add structured fields to log events (provider, model, iteration, tool name)
• Keep user-facing output (println!) separate from logs
• Logs go to stderr, user output to stdout

Relates to

• feat: add debug mode to show raw LLM request/response data #38 (debug mode to show raw LLM data) — logging infra enables this