Add mapbox config; first setting persists the update-check opt-out

[mattpodwysocki]

Summary

• MAPBOX_NO_UPDATE_CHECK=1 only lasts for the shell session it's set in — there was no way to turn the update check off once and have it stay off.
• Adds mapbox config get|set update-check on|off, persisted to ~/.mapbox/config.json (or $MAPBOX_CONFIG_DIR), written through the same write_private credentials use (0600).
• update_check::enabled gains a fifth gate alongside the existing four, checked at both call sites (notify() and the detached refresh child) — either the env var or the persisted setting is enough to stop it.
• get/set take a key restricted to a small table, so a second setting is a new key and match arm rather than a new pair of subcommands.

Test plan

• cargo build, cargo fmt, cargo clippy --all-targets -- -D warnings, cargo test all clean
• New unit tests in src/config.rs (default value, round-trip)
• New tests/config.rs: get/set persistence across separate invocations, unknown key/value is a usage error, file permissions
• New e2e tests in tests/update_check.rs: the persisted opt-out stops the refresh child and silences the terminal notice, exercised through the real mapbox config set rather than a hand-seeded file
• docs/commands.md and README updated; CHANGELOG entry added

[zmofei]

Nit (non-blocking): in notify(), crate::config::update_check_enabled() is passed as an argument to enabled(). Rust computes all function arguments before the call, so this reads config.json on every run — even when stderr isn't a terminal, or MAPBOX_NO_UPDATE_CHECK is set. Those cases used to cost nothing. Suggestion: make the last parameter of enabled() a closure instead of a plain bool, so the && chain only reads the file when the earlier, cheaper checks already passed.