Follow the Rust style guide's smaller rules (#353)

Measured against /src/styleguide/rustguide.md:
- Each use statement imports from one module, and free functions are
  called through theirs (db::step, db::now, jsonwebtoken::decode,
  crate::feed::redact, unix::signal) rather than imported bare.
- unsafe_code is forbidden workspace-wide. The one unsafe was a test's
  set_var, which is unsound while other tests' threads read the
  environment; the test now reads PATH, which every run has.
- Locks say why they may panic: expect("a panic elsewhere poisoned this
  lock") in place of 25 unwraps, and unique_slug's endless search says
  it cannot end.
- No combinator chains in for headers: each is a named variable first.
- #[must_use] on the 27 functions clippy's must_use_candidate names.
- A literal bool passed where its meaning is not plain gets its name
  (/* keep_open */ true, /* piece */ false, ...).

No behaviour changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/directory/src/config.rs

@@ -123,18 +123,21 @@ impl Config {
 }
 
 /// A token's SHA-256 in hex, as the config file holds it.
+#[must_use]
 pub fn hash(token: &str) -> String {
     use sha2::Digest;
     sha2::Sha256::digest(token.trim().as_bytes()).iter().map(|b| format!("{b:02x}")).collect()
 }
 
 /// Equal without saying, by how long it took, how much of the guess was right.
+#[must_use]
 pub fn same_secret(a: &str, b: &str) -> bool {
     let (a, b) = (a.as_bytes(), b.as_bytes());
     a.len() == b.len() && a.iter().zip(b).fold(0u8, |acc, (x, y)| acc | (x ^ y)) == 0
 }
 
 /// A new token: `ipxd_` and 256 random bits, the prefix so one found in a log says what it is.
+#[must_use]
 pub fn new_token() -> String {
     use std::io::Read;
     let mut bytes = [0u8; 32];