fix(engine): verify resumed worktrees aren't bootstrap-misbound

The resume path in acquireTaskWorktree returned a reused worktree
without checking whether its branch contained foreign commits. If a
sibling task's tip had been baked into the branch at creation time,
the executor preflight would later fail contamination checks forever
(observed in the FN-5475 cascade).

The resume path now computes a fresh merge-base and runs
classifyBootstrapMisbinding. For the foreign-only / zero-own-commits
shape it re-anchors inline and emits a branch:reanchor audit event.
Mixed contamination continues to flow through the executor's
primary recovery path.

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

.changeset/fix-engine-resume-misbinding-verify.md

@@ -0,0 +1,24 @@
+---
+"@fusion/engine": patch
+---
+
+fix(engine): verify resumed worktree branches aren't bootstrap-misbound
+
+`acquireTaskWorktree` short-circuited the resume path when
+`task.worktree` existed on disk and classified `ok`, handing the
+worktree back to the executor without inspecting its branch history.
+If the branch had been created from a poisoned local-main tip (a
+sibling task's commit), the executor preflight would later flag every
+intermediate landing as foreign and the task would loop through
+contamination recovery until pausing for human adjudication
+(observed in the FN-5475 cascade).
+
+The resume path now computes a fresh merge-base against local `main`
+(falling back to `origin/main`) and runs `classifyBootstrapMisbinding`
+on the branch. When the range is purely foreign with zero own commits,
+it re-anchors the branch inline via `reanchorBranchToBase` and emits a
+`branch:reanchor` audit event with `trigger: "resume-misbinding"`.
+
+Mixed contamination (own + foreign, or non-attributed commits) is
+deliberately left to the executor's existing primary path so the
+richer adjudication flow still applies.