fix: replace .unwrap() with .expect() on infallible calls

[arhtur007]

Description

Replace four .unwrap() calls with .expect("...") so the panic message documents the invariant.

Changes

• src/config.rs — expand_env_vars regex literal
• src/discord.rs — sender_ctx JSON serialization, strip_mention regex, shorten_thread_name GitHub URL regex

Rationale

All four call sites are infallible in practice (regex literals known at compile time, and a serde_json::Value built from a json! macro). .expect("reason") is the conventional form
for "should never panic" — if it ever does fire, the message points at the violated invariant.
.unwrap_or_else() is not suitable here because there is no meaningful fallback value for an invalid regex or a failed serialization — the program cannot continue correctly, so a panic
with a descriptive message is the right behavior.

[masami-agent]

LGTM — straightforward improvement. All four sites are infallible (compile-time regex literals and json! macro output), so .expect() with a reason string is the idiomatic Rust convention here. Good housekeeping 👍