Default new worktrees to the upstream branch (#2848)

* fix(workspaces): default new worktrees to upstream refs

Use the daemon-resolved upstream as the untouched starting ref so unpushed local commits do not leak into new workspaces. Preserve exact base identity for fork remotes while keeping local-first behavior across version drift.

* fix(workspaces): preserve exact base refs in git actions

Resolve accepted display aliases back to the stored ref before comparisons and updates. Fail closed when a non-origin remote base has no explicit mutable merge target.

* fix(workspaces): reject conflicting qualified base refs

Display aliases remain accepted for compatibility, but a qualified ref carries remote identity and must not be canonicalized to a different stored upstream.

* test(server): make upstream fixtures portable

Keep the checkout-status fixture aligned with the optional upstream field, and select main explicitly when cloning a bare test remote whose default HEAD varies by platform.

* fix(workspaces): preserve exact upstream refs

Qualified remote refs now fail closed when pruned instead of switching lineage. Metadata accepts Git-valid upstream names rather than applying a narrower character whitelist.

* fix(workspaces): preserve qualified local base refs

Local tracking branches are exact upstream identities too. Keep heuristics only for legacy unqualified metadata so origin cannot replace a stored refs/heads base.

packages/server/src/utils/worktree.ts

@@ -1271,6 +1271,7 @@ export const createWorktree = async ({
 
   writePaseoWorktreeMetadata(worktreePath, {
     baseRefName: sourcePlan.metadataBaseRefName,
+    ...(sourcePlan.metadataBaseRef ? { baseRef: sourcePlan.metadataBaseRef } : {}),
     ...(sourcePlan.changeRequestLookupTarget
       ? { changeRequestLookupTarget: sourcePlan.changeRequestLookupTarget }
       : {}),
@@ -1300,7 +1301,11 @@ interface ResolveWorktreeSourcePlanOptions {
 
 interface WorktreeSourcePlan {
   branchName: string;
+  // Display name and exact ref are two different facts. The name cannot round-trip to a
+  // commit — "main" resolves local-first even when the worktree was cut from a fork's
+  // upstream — so comparisons and actions read the ref and the UI reads the name.
   metadataBaseRefName: string;
+  metadataBaseRef?: string;
   changeRequestLookupTarget?: PaseoWorktreeChangeRequestHint;
   addArguments: string[];
   pushRemote?: {
@@ -1334,6 +1339,7 @@ async function resolveWorktreeSourcePlan({
       return {
         branchName: newBranchName,
         metadataBaseRefName: normalizedBaseBranch,
+        metadataBaseRef: resolvedBaseBranch,
         addArguments: ["-b", newBranchName, "--no-track", base],
       };
     }