feat: add land command for merging stacked PRs

[luqven]

Summary

Adds a new land subcommand that intelligently merges a stack of PRs using the spr/Graphite optimization pattern, plus automatic repository detection from git remotes.

How it works

1. Finds the topmost PR where all PRs below it are approved
2. Updates that PR's base to the target branch (main/master)
3. Squash-merges that single PR (contains all commits from the stack)
4. Closes all PRs below it with a "Landed via #N" comment

Usage

# Auto-detects repository from git remote - no -r flag needed!
gh-stack land PROJ-123

# Preview what would happen
gh-stack land PROJ-123 --dry-run

# Skip approval checks
gh-stack land PROJ-123 --no-approval

# Only land bottom 2 PRs
gh-stack land PROJ-123 --count 2

# Use a different remote (default: origin)
gh-stack land PROJ-123 --origin upstream

# Override auto-detection with explicit repo
gh-stack land PROJ-123 -r owner/repo

Features

Land Command

• --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
• Clear error messages with hints for resolution

Auto-detect Repository (NEW)

• Automatically detects owner/repo from git remote URL
• Works with SSH and HTTPS URLs (including GitHub Enterprise)
• Fallback chain: -r flag → GHSTACK_TARGET_REPOSITORY env var → auto-detect
• --origin / -o flag to specify which remote to use (defaults to "origin")
• Prints detection message: Detected repository: owner/repo (from origin remote)
• Applied to all commands: annotate, log, autorebase, land

Changes

• New src/land.rs module with landing logic
• New src/api/land.rs module with GitHub API methods for merge/close/update
• Added land subcommand to CLI in main.rs
• Added parse_github_remote_url() and detect_repo_from_remote() in src/tree.rs
• Added resolve_repository() helper with fallback chain in main.rs
• Added --origin flag to annotate, log, autorebase, and land commands
• Added serial_test dev dependency for API mock tests
• 9 new unit tests for land planning logic
• 9 new unit tests for URL parsing
• 3 new integration tests for API methods