fix: correct git stash message argument and format_relative_time calendar dates

[JiayuuWang]

Summary

Two bug fixes in cc-core utilities (src-rust/crates/core/src/):

1. git_utils::stash() — incorrect argument splitting

The stash message was formatted as a single string "-m hello world" and passed via args(). Rust's Command::args() passes each element as a separate OS-level argument, so "-m hello world" was handed to git as one token — git would reject it as an unknown flag and the stash would silently fail.

Before:

let msg_flag = format!("-m {}", m);
args.push(&msg_flag);          // git sees: ["stash", "push", "-m hello world"]

After:

cmd.arg("-m").arg(m);          // git sees: ["stash", "push", "-m", "hello world"]

2. format_relative_time() — "X days ago" instead of calendar date

The doc-comment promises "Mar 15" for timestamps older than 2 days, but the else branch returned "X days ago" instead. Fixed by computing month/day from the UNIX epoch using Howard Hinnant's civil-calendar algorithm (no new dependencies needed).

Tests added for both fixes.

Test plan

• cargo test -p cc-core — all existing tests pass; new tests added for days_to_month_day and format_relative_time_old_timestamp_returns_calendar_date
• Manual: stash with a message containing spaces now works correctly