Description

Layer 2 of 5, stacked on #22971. The base is sergiou87-assisted-commit-ux at b79922ca5ad766f3bee0cbf13dbfdd8e76e8a35b, which includes development at bebc062caafcf818993b9f53397b976d33484231; this PR still targets the preceding layer, not development directly.

⚠️ For a better understanding of this PR (and the whole feature) I recommend checking the docs introduced in this PR, mainly docs/technical/assisted-commit-process.md ⚠️

This adds the standalone plan-first transaction backend. Desktop captures selected content, validates the entire proposed plan and every cumulative tree, then creates only the planned local commits. Copilot receives opaque snapshot change IDs; it cannot supply executable paths, patches or blobs, and this layer contains no SDK/model calls.

• Immutable selected-only snapshots preserve file/line selections, raw bytes, type/mode, original full HEAD/ref and real index. Selected text hunks from the same file can be committed separately in either order; indivisible changes are atomic or fail with a documented unsafe condition.
• Whole-plan validation enforces exact-once exhaustive ownership and rejects unknown/injected fields, IDs, duplicates, omissions, blank titles, empty groups and unintended empty deltas. All cumulative trees are prepared before the first published commit and the final tree must equal exactly the frozen selection. Single-commit fallback requires an explicit whole-selection message/reason; invalid payloads never silently succeed.
• Capture uses isolated indices/configuration/attributes and owned copies of selected bytes for native index-aware conversion. The real index remains untouched during analysis/execution. Native clean/process filters retain original-root context, and ordinary Git's indexed CRLF semantics are preserved.
• Guarded local execution reuses message/trailer/sign-off/skip-hook and native hook-interception conventions for every commit. Cooperative cancellation waits for in-flight Git. Ref CAS, complete HEAD identity, parent/tree/index verification and parent-owned locks protect unrelated history and staging.
• Recovery removes only this run's commits, preserves current working-tree bytes and original index, exposes original-selection reconciliation data, and retains explicit retry/finalization capabilities when interference or resource cleanup blocks recovery. No hard reset, checkout, stash, amend, push or existing-history rewrite.
• Failed snapshot capture or pre-execution disposal exposes an opaque cleanup-only token through AssistedCommitError.cleanupToken. retryAssistedCommitCleanup retries removal of identity-checked private temporary resources without reading or changing HEAD, the index or working-tree files. Repeated failures retain the same capability; successful cleanup is idempotent. Existing executor rollback/recovery tokens and original failure causes remain intact.

Public APIs and lifecycle/support/unsafe cases are documented in docs/technical/assisted-commit-transactions.md. Original manual commit/staging/parser behavior and layer 1's availability/callback gate remain unchanged. Shared existing helpers only gain optional environment plumbing; unrelated issues already present in development are not fixed.

Scope boundary: backend scaffolding only. The constrained planner belongs to layer 3; AppStore/Dispatcher/UI run/progress/cancel/hook recovery wiring belongs to layer 4; opt-in push/retry belongs to layer 5. No real assisted callback is wired here.

Stack

Merge in order, starting with 1. Each PR builds on the preceding PR.

1. #22971 - Repository-scoped assisted commit UX
2. #22976 - Selected snapshots and validated Git transactions (this PR)
3. #22977 - Constrained Copilot-assisted commit planning
4. #22986 - Awaited local runs and recovery
5. #22991 - Optional push and push-only retry

Validation

The four plain-suite bare-fixture failures (three push fixtures and one bare-repository detection fixture) are caused by the CLI-injected safe.bareRepository=explicit policy. They were independently reproduced from an archive of exact pinned parent b79922ca5ad766f3bee0cbf13dbfdd8e76e8a35b, without this layer; the same four parent tests pass with the process-only override. No global or repository configuration was changed. These baseline issues remain out of scope and unfixed. The former dependency declaration conflicts are resolved upstream; the current plain strict TypeScript check passes.

There are 155 new real-Git/fault tests beyond the lower layer. Coverage includes partial/full selections and same-file offset/reordering, actual trees/full SHAs/counts and working-tree/index byte equality, binary/rename/delete/new/mode/path/EOL cases, hostile plans, selected versus unselected mutation, cancellation boundaries/in-flight Git, real declined/ignored hooks, unintended hook staging, ref/index interference, unborn/detached/linked worktrees, and repeatable owned-resource cleanup/recovery. Cleanup-specific cases cover capture failures before a snapshot exists, planner/validation/cancellation failures, repeated disposal failures, idempotence, forged capabilities, active-operation exclusion and replaced temporary roots without changing repository content.

Screenshots

Not applicable: this layer changes no UI surface and wires no assisted executor callback.

Release notes

Notes: no-notes