fix(logging): restore RUST_LOG env var support in init_logging

[thedavidweng]

Summary

Restore RUST_LOG environment variable support in init_logging() by replacing hardcoded filter_level(Info) with Builder::from_env(). Default filter remains info when RUST_LOG is not set.

Motivation

The current implementation ignores RUST_LOG entirely, contradicting DESIGN.md §25.3:

Rust core uses env_logger which outputs to stderr. To view logs, run the app from terminal with RUST_LOG=info.

This makes it impossible to enable debug-level logging without modifying source code.

Changes

• koe-core/src/telemetry.rs: init_logging() now uses env_logger::Builder::from_env(Env::default().default_filter_or("info")) instead of builder().filter_level(Info)

Behavior

Scenario	Before	After
No RUST_LOG set	info level	info level (unchanged)
RUST_LOG=debug	info level (ignored)	debug level ✅
RUST_LOG=koe_core=debug	info level (ignored)	debug for koe_core only ✅

Testing

• cargo check --no-default-features -p koe-core — ✅
• cargo clippy --no-default-features -p koe-core — ✅ (1 pre-existing warning, unrelated)
• cargo test --no-default-features -p koe-core — ✅ 44 tests pass
• cargo fmt --check — ✅