fix(FN-5456): never create task branches from arbitrary HEAD in autocorrect

`attemptBranchAutocorrect` was the only branch-creation site in the
engine that ran `git checkout -B <expected>` without a start point.
When the worktree's HEAD was at a previous occupant's tip, the new
label silently captured that commit — the "branch: Created from HEAD"
contamination pattern that the cross-contamination guard then refuses
to auto-resolve (see FN-5456: orphan FN-5477 commit 268574b9a stranded
on fusion/fn-5456).

Replace the unsafe fallback with verify-then-`git checkout`: only switch
to an existing expected ref; return `failed` when it does not exist so
upstream recovery — which knows the proper base SHA — can re-anchor via
`prepareForTask` / `reanchorBranchToBase`.

Tests updated for the new command sequence; new case covers the
"expected ref missing → fail without creating from HEAD" guarantee.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/fix-fn-5456-branch-autocorrect-no-create-from-head.md

@@ -0,0 +1,22 @@
+---
+"@fusion/engine": patch
+---
+
+fix(engine): never create task branches from arbitrary HEAD in autocorrect
+
+`attemptBranchAutocorrect` previously fell back to `git checkout -B
+<expected>` with no start point when rename was not applicable. If the
+worktree's HEAD happened to be at a previous occupant's commit (e.g. an
+orphaned tip from a different task), the new branch label silently
+captured that commit — the "branch: Created from HEAD" contamination
+pattern that the cross-contamination guard then refuses to auto-resolve.
+
+This is the only branch-creation site in the engine that did not thread
+a resolved base SHA; every other path (`prepareForTask`,
+`reanchorBranchToBase`) already passes the base explicitly.
+
+Autocorrect now verifies the expected ref exists and uses a plain
+`git checkout`, so it can only *switch to* an already-existing branch.
+When the ref is missing it returns `failed`, letting upstream recovery
+(which knows the proper base) re-anchor with `prepareForTask` /
+`reanchorBranchToBase`.