FN-9053: prevent workspace acquisition from persisting singular worktrees

Keep workspace tasks classified as multi-repository throughout sub-repository worktree acquisition.

- Suppress singular worktree and branch persistence for workspace sub-repo acquisition.
- Preserve the single-repository persistence contract and add regression coverage for acquisition failures.
- Document the workspace acquisition-state invariant and add a patch changeset.

Files changed:
 .../fn-9053-workspace-acquisition-atomicity.md     |  7 ++
 docs/architecture.md                               |  1 +
 .../worktree-acquisition-workspace.test.ts         | 98 ++++++++++++++++++++--
 .../engine/src/worktree/worktree-acquisition.ts    | 66 +++++++++------
 4 files changed, 142 insertions(+), 30 deletions(-)

Fusion-Task-Id: FN-9053

Fusion-Task-Lineage: e27d1e90-fc99-4bb6-ac83-033f71abc521

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

docs/architecture.md

@@ -626,6 +626,7 @@ See [Memory Plugin Contract](./memory-plugin-contract.md) for the full plan.
 ### Agent roles
 - **Planning**: the planning processor generates task plans (`PROMPT.md`) and selects eligible planning tasks by priority first, then FIFO (`createdAt` ascending) within each priority tier. Each attempt captures the authoritative artifact baseline and owns its fallback callback provenance. Only a settled, fallback-free attempt that changed that exact baseline and passes deterministic validation may hand off to workflow Plan Review. Empty, unchanged, or fallback-engaged attempts use the shared bounded `recoveryRetryCount`/`nextRecoveryAt` backoff; exhaustion persists an actionable planning error and never signals successful handoff. After a prompt settles, triage awaits the originating runtime's finite `settleFallbackDispatch` lifecycle signal, then awaits every observer callback admitted by that signal before deciding. A configured runtime that cannot supply this signal fails closed through the same bounded planning recovery rather than handing a potentially fallback-authored plan to review. This deliberately never inspects arbitrary Node timers: clean planner housekeeping can schedule unrelated one-shot or recurring timers without delaying admission. A callback from an obsolete attempt remains scoped to that attempt. Explicit duplicate-marker closure runs only after this same clean-attempt admission. If the stuck-task detector kills a not-yet-approved planning session after a non-empty `PROMPT.md` draft exists, the retry is requeued as `needs-replan` and seeds the next prompt in revision mode from that draft instead of cold-starting. A newly added dependency in a hold lane follows the same durable `needs-replan` path; it never clears status, so a planner interrupted after prompt persistence remains claimable and cannot silently bypass the approval/release handoff. When `PROMPT.md` is absent, a non-empty `plan` task document written through `fn_task_document_write` is the fallback seed; missing or whitespace-only drafts still cold-start.
 - **Executor**: `TaskExecutor` (`executor.ts`) implements tasks in worktrees
+  - **Workspace acquisition shape:** per-repo acquisition persists only `workspaceWorktrees`; it never exposes a sub-repo path or branch through singular `task.worktree`/`task.branch`, including if the final workspace-state write fails. This preserves workspace classification for dashboard rendering, self-healing, and executor dispatch.
   - **Task-pinned orphan recovery:** task-ID-pinned acquisition holds one path reservation across classification, preservation, quarantine reconciliation, and recreation. Inactive incomplete or unregistered directories are atomically moved to `<project>/.fusion/recovery/worktrees`, or to `<worktreesDir>/.fusion-recovery/worktrees` after an `EXDEV` cross-filesystem refusal. Each actual recovery root retains the newest 10 recognized Fusion-generated entries; pruning is fail-soft and preserves unknown, symlinked, unreadable, or active paths. Worktree pool and self-healing scans exclude both `.ai-merge` and `.fusion-recovery` as internal container boundaries.
   <!-- FNXC:MergerUnification 2026-08-09-12:04: Master-plan U0 made clean-room `runAiMerge` the sole production merge path. The legacy `aiMergeTask` auto-prerebase policy is retained but inert, so executor reused-base refresh must not describe it as live merger behavior. -->
   - **Execution-only reused-base refresh (FN-8693):** planning creates isolated worktrees but does not refresh them; immediately before a graph `code` node, normal executor dispatch, or durable-agent heartbeat session, refresh-enabled reuse resolves the current integration target C1 and compares it with durable `task.baseCommitSha`. A clean no-own-commit checkout resets to C1; a clean own-commit checkout rebases and retains its resulting C2 `HEAD`, while storing C1—not C2—as the baseline. A durable C0/C1 mismatch is rechecked from git and durable metadata on every acquisition, so restart reconciliation needs no in-memory marker. Dirty, unresolved, unsupported worktrunk, git, conflict, persistence, and unprovable-reconciliation cases are typed non-execution outcomes that park before session start. If baseline persistence fails after git moves `HEAD`, the engine compensates to the original clean checkout and emits `worktree:base-refresh-persistence-failed-compensated`; otherwise it requires later proof-based reconciliation. Audit events are `worktree:base-refreshed`, `worktree:base-refresh-blocked`, `worktree:base-refresh-conflict`, `worktree:base-refresh-persistence-failed-compensated`, and `worktree:base-refresh-reconciled`. Plan/review/gate acquisition and merger acquisition remain excluded; production merger behavior is the unified clean-room `runAiMerge` path.