feat: add local merge (ez merge --local) and fold (ez fold) commands

[dezren39]

Adds ez merge --local for merging branches without GitHub and ez fold for collapsing branch ranges. Also adds --remote flag to push/submit, fixes cross-fork PR detection, and handles "already exists" errors gracefully.

Changes

Local merge and fold

• ez merge --local — merge current branch into parent locally
  • --strategy squash (default), --strategy rebase, --strategy merge
  • --into <branch> — merge into a specific branch instead of parent
  • --force — suppress merge-commit warning
• ez fold <range> — fold a range of branches into one
  • ez fold feat/a..feat/c — squash range, survivor is bottom branch
  • --name — rename surviving branch
• Reparents child branches and cleans up worktrees on merge/fold
• Git helpers: merge_squash, merge_no_ff, rename_branch, update_branch_ref

Fork workflow fixes

• --remote flag on ez push and ez submit — push to a specific git remote, stored per-branch on first use
• Cross-fork PR detection — get_pr_status_in_repo now retries with fork-prefixed head (e.g. dezren39:feat/config) when the bare branch name returns no PR in a cross-fork repo
• "Already exists" handling — when gh pr create fails because a PR already exists, ez now parses the PR URL from the error, links it, and continues instead of failing

Commits (3)

• 8d66501 feat: add local merge (ez merge --local) and fold (ez fold) commands
• 32f3dce test: add merge/fold CLI parse tests, merge_squash, rename_branch, update_branch_ref git helper tests
• 1e6facd feat: add --remote flag to push/submit, fix cross-fork PR detection and already-exists handling

Resolves rohoswagger#6

259 tests pass, 0 failures.

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Stack (this PR is 5 of 5):

1. feat/config (rohoswagger#9)
2. feat/preflight (dezren39#1)
3. feat/worktree-ops (dezren39#2)
4. feat/fork (dezren39#3)
5. → feat/local-merge ← you are here (dezren39#4)