refactor(deps): remove unused dependencies

[Nickamoto]

Remove dependencies confirmed unused by cargo-machete analysis, as identified in the dependency audit (discussion #8, issue #29).

mujina-miner:

• sha2: bitcoin crate provides SHA-256 via bitcoin_hashes
• hyper (direct): still available transitively via axum/reqwest
• modular-bitfield: no usage in source

mujina-dissect:

• hex: no usage in source
• thiserror: no usage in source
• tracing: no usage in source (tracing-subscriber retained)

Also removes sha2, hyper, and modular-bitfield from workspace dependencies as they are no longer referenced by any member.

Fix unused import warnings in tracing.rs by gating linux-only imports (env, tracing_journald, prelude) behind cfg(target_os).

Eliminates ~15 exclusive transitive crates.

Refs: #29

[jayrmotta]

tACK e8718fc

[average-gary]

Independently arrived at the same changes via a dependency audit posted in Discussion #8. Confirmed the same 6 unused deps using both cargo-machete (source-code heuristic) and cargo +nightly udeps (compiler-level analysis):

$ cargo machete
mujina-dissect -- ./tools/mujina-dissect/Cargo.toml:
	hex
	thiserror
	tracing
mujina-miner -- ./mujina-miner/Cargo.toml:
	hyper
	modular-bitfield
	sha2

$ cargo +nightly udeps --workspace
unused dependencies:
`mujina-miner v0.1.0`
└─── dependencies
     └─── "modular-bitfield"

Nice that this PR also fixes the tracing.rs cfg gating — we noted those as pre-existing clippy failures on macOS but didn't include the fix. LGTM.

Closed my duplicate PR #35 in favor of this one.

[johnny9]

What happens with this on macos?

[skot]

I believe the use tracing_journald; line is skipped

[Nickamoto]

skot is right. Also updated Cargo.toml, moving tracing-journald into [target.'cfg(target_os = "linux")'.dependencies], so it won't be compiled on macOS at all rather than just skipped.

[rkuester]

Thanks for your first contribution, Nick, and welcome to the project!

I restructured the commits slightly. The original first commit mixed dep removal with cfg-gating fixes in tracing.rs. I moved the cfg-gating changes into the second commit alongside the tracing-journald platform gating.

Also changed the type from refactor to build since these are build configuration changes rather than code restructuring with no change in behavior.