feat: add land command for merging stacked PRs

Add a new 'land' subcommand that intelligently merges a stack of PRs:
- Finds the topmost PR where all PRs below it are approved
- Updates that PR's base to the target branch (main/master)
- Squash-merges that single PR (contains all commits from the stack)
- Closes all PRs below it with a 'Landed via #N' comment

Features:
- --dry-run: Preview what would happen without making changes
- --no-approval: Skip approval requirement check
- --count N: Only land bottom N PRs in the stack
- Draft PRs block landing of PRs above them

This follows the spr/Graphite optimization pattern for efficient stack landing.

Author: luqven

Cargo.lock

@@ -30,4 +30,5 @@ chrono = { version = "0.4", features = ["serde"] }
 [dev-dependencies]
 insta = "1.39"
 mockito = "1.4"
+serial_test = "3.1"
 tokio-test = "0.4"