fix: remove stale attribute, add Config docs, fix doc links, remove dead code

[PrimeFactor-Dev]

Closes #686, Closes #679, Closes #688, Closes #684

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Documentation update

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Summary

Four low-risk documentation and dead-code cleanup fixes across the oracle and shared-config crates:

1. Stale #[allow(unused_mut)] attribute on a non-mut listener binding in main.rs #686: Remove stale #[allow(unused_mut)] attribute from main.rs — the listener is bound with plain let, so the attribute is a no-op leftover from an earlier version.
2. Config struct: 21 of 24 public fields have no doc comments #679: Add one-line /// doc comments to all 21 undocumented public fields in the Config struct, including env var names and defaults.
3. EnvErrors doc comment links to Config::from_lookup, which is a private function #688: Fix EnvErrors doc comment that linked to private Config::from_lookup (unresolvable in external docs). Removed the link to from_lookup.
4. shared_config exposes MissingEnvVar, load_from_env_var, load_from_file, and build_lookup — none are called outside their own unit tests #684: Remove unused public items from shared_config: ConfigError::MissingEnvVar (never constructed), load_from_env_var, load_from_file, and build_lookup — none have call sites outside their own unit tests.

Motivation / Context

These are four tracked issues addressing dead code, missing documentation, and broken doc links. Each fix is minimal and scoped to the exact problem described in the issue.

Closes #686, Closes #679, Closes #688, Closes #684

Testing

All verification gate commands passed:

cargo fmt --all -- --check                    ✓
cargo clippy --all-targets --all-features --locked -- -D warnings  ✓
RUSTFLAGS=-Dwarnings cargo test --all --locked  ✓ (335 tests passed)

Tradeoffs

• The MissingEnvVar variant removal is a minor API change to shared_config, but since no code constructs or matches on it outside tests, this is safe.
• Doc comments on Config fields use env var names derived from the from_lookup implementation — these are the authoritative source.

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