chore: remove unused dependencies identified by cargo-machete and cargo-udeps

[average-gary]

Context

Per Discussion #8, a dependency audit identified 6 direct dependencies with zero usage in source code.

Unused Dependencies

mujina-miner

• sha2 — the bitcoin crate provides SHA-256 via bitcoin_hashes; sha2 is never imported
• hyper (direct) — only used transitively via axum/reqwest, never imported directly
• modular-bitfield — never imported anywhere in source code

mujina-dissect

• hex — never imported
• thiserror — never imported
• tracing — never imported

Tool Evidence

cargo-machete (source-code heuristic, stable Rust)

$ 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 (compiler-level analysis)

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

cargo-udeps only flags modular-bitfield because sha2 and hyper are still compiled as transitive dependencies of other crates. Removing them as direct dependencies is still correct — they remain available transitively for the crates that actually need them.

Proposed Change

Remove all 6 entries from the respective Cargo.toml files. Zero source code changes needed. Eliminates 15 exclusive transitive dependencies from the build.

[average-gary]

Closing as duplicate of #29, which covers the same scope.