feat: implement per-project GitHub sync scheduler with dynamic intervals

Replaces fixed-interval polling with a priority queue scheduler that adjusts
check frequency based on activity recency (15min for active projects, up to
24hr for stale ones). Includes exponential backoff on errors and staggered
initial checks to prevent API rate limit issues.

Cargo.lock

@@ -116,6 +116,7 @@ dependencies = [
  "mime_guess",
  "moka",
  "nu-ansi-term",
+ "parking_lot",
  "rand 0.9.2",
  "reqwest",
  "serde",