Delete merged worktrees: count PRs merged into a stack parent or release branch

[leomontigatti]

Follow-up to #14758, which was closed with a pointer to Ideas.

The problem

I work in stacked PRs: each layer's PR targets the branch of the layer below it, not main. When a layer merges into its parent, Delete merged worktrees keeps its worktree. storageCleanupPullRequestMerged (apps/server/src/storageCleanup.ts) accepts a merged PR only when pullRequest.baseRef === worktree.defaultBranch (or when the head is already in the default branch). So a layer's worktree waits until the whole stack lands in main, and with a squash merge of the parent it waits forever. Today I clean these up by hand.

Proposal

Count a PR as merged for cleanup whatever its base branch is, as long as the host says it is merged and its headRef and headSha match the worktree's branch and head. That is the exact-head check cleanup already makes for squash merges into the default branch, minus the base-branch condition.

I think this is safe for the same reason the squash case is: cleanup keeps the local branch (and T3 recreates the checkout from it on resume), and the commits now live on the parent's branch on the remote. Removing the worktree loses no work. If an unrestricted base feels too broad, an opt-in (for example a "PRs merged into any branch" switch under Storage) would cover it too.

A related case with the same fix: a PR that targets a release branch that hasn't been merged back into main yet.