fix(engine): un-deadcode bootstrap-misbinding auto-recovery fallback

The auto-recovery handler in branch-worktree.ts passed
foreignCommits: [] to classifyBootstrapMisbinding, and the classifier
gated isBootstrapMisbinding on foreignCommits.length > 0. The entire
reanchor block was dead code on this path — the FN-5475 cascade hit
"human adjudication" instead of recovering.

The classifier now derives the foreign-commit count from its own
git log walk; the input field is advisory/optional. Result type gains
foreignCommitCount. The fallback handler also stops using
ctx.task.baseCommitSha (deliberately stale per FN-4417) and computes
a fresh merge-base against local main / origin/main, matching the
executor's primary contamination path.

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

.changeset/fix-engine-bootstrap-misbinding-fallback.md

@@ -0,0 +1,28 @@
+---
+"@fusion/engine": patch
+---
+
+fix(engine): un-deadcode the bootstrap-misbinding auto-recovery fallback
+
+The auto-recovery handler in `auto-recovery-handlers/branch-worktree.ts`
+called `classifyBootstrapMisbinding` with `foreignCommits: []` because it
+had no `BranchCrossContaminationError` in hand (it discovers the conflict
+via `inspectBranchConflict`). The classifier's predicate gated on
+`foreignCommits.length > 0`, so the input always resolved to
+`isBootstrapMisbinding: false` and the re-anchor block was effectively
+dead code.
+
+The handler also used `ctx.task.baseCommitSha` as the contamination base,
+which is deliberately preserved across sessions for diff math (FN-4417)
+and can lag local `main` by many commits — causing legitimately-merged
+landings to be classified as foreign at this layer.
+
+Changes:
+- `classifyBootstrapMisbinding` now derives the foreign-commit count from
+  its own `git log baseSha..branchName` walk; `input.foreignCommits` is
+  optional and advisory only. The result type gains `foreignCommitCount`.
+- The `branch-worktree` recovery handler stops passing an empty array and
+  computes a fresh merge-base against local `main` (falling back to
+  `origin/main`), mirroring the executor's primary contamination path.
+- Regression tests cover both the no-`foreignCommits` call shape and the
+  `foreignCommitCount` field.