[FEAT] back SpaceDO filesystem with Cloudflare Artifacts

[karishnu]

SpaceDO gives each app an isolated, git-backed workspace running entirely on the Workers runtime. Previously, the workspace depended on Durable Object-local filesystem state, with no durable remote source of truth for an app’s committed history.

This PR makes Cloudflare Artifacts the durable, git-compatible source of truth for each app while preserving SpaceDO’s existing filesystem API.

How it works

The Artifacts-backed filesystem implements the same FileSystem interface used by the rest of SpaceDO. It combines:

• Writable overlay: An in-memory filesystem containing active edits, hydrated files, and local git objects.
• Base snapshot: A read-only index of the current Artifacts branch.
• Whiteouts: Tombstones that prevent deleted base files from reappearing.

flowchart LR
  A[Artifacts repository] -->|fetch branch| B[Local git object store]
  B --> C[Read-only base index]
  C -->|hydrate on demand| D[Writable in-memory overlay]
  D -->|commit and push| A

Loading

Reads and hydration

Point reads first check the writable overlay. If a file exists only in the base snapshot, its blob is read from the local git object store and hydrated into the overlay.

Deleted base files remain hidden through whiteouts.

The filesystem provides two readiness levels:

• ready() ensures the Artifacts snapshot is loaded and indexed.
• whenFullyMaterialized() hydrates every visible base file into the overlay.

Full materialization happens before whole-tree operations such as commits, deploys, rollbacks, recursive listings, searches, and workspace statistics.

Writes and deletes

• Writes and edits are applied to the overlay.
• Writing a previously deleted path clears its whiteout.
• Deleting a base file records a whiteout.
• Directory deletion and moves whiteout all affected base descendants.
• Copy and move operations hydrate base-only sources before modifying them.

Cold-start recovery

The live filesystem and local git repository are in memory. When a SpaceDO instance starts:

1. It restores the app’s persisted Artifacts remote URL from Durable Object storage.
2. It fetches the app’s branch into the in-memory git repository.
3. It walks the fetched commit tree to rebuild the base index.
4. It hydrates files into the working tree as they are accessed.

An Artifacts repository with no commits is valid and represents an empty base. Until the first commit, the filesystem behaves as a normal writable in-memory workspace.

Artifacts synchronization

Artifacts synchronization manages repository provisioning, authentication, fetches, and pushes.

This PR improves synchronization reliability through:

• Durable remote discovery: The Artifacts remote URL is persisted in Durable Object storage and reused after cold starts.
• Serialized pushes: Concurrent commits and deploys within one SpaceDO cannot race each other.
• Bounded retries: Stale-ref and concurrent-update failures are retried with backoff.
• Token refresh: Write tokens are cached and refreshed before expiration.
• Best-effort failure handling: Sync errors are logged and returned as failures instead of causing commit or deployment RPCs to throw.

ARTIFACTS is a required binding. There is no local-only git fallback.

Local development and tests must provide an ARTIFACTS binding.

Commit, deploy, preview, and rollback

• Commit: Materializes the complete tree, creates a git commit, and mirrors the branch to Artifacts.
• Deploy: Builds from the complete overlay-aware filesystem before pushing the commit.
• Preview: Uses the correct filesystem when assembling deployment files, including files restored from Artifacts.
• Rollback: Fetches Artifacts before resolving the target commit, allowing rollback even when the current SpaceDO has never held that commit locally.
• Internal paths: /.git and /.afs are excluded from user-visible listings, deployment bundles, and rollback trees.

Editor synchronization fixes

This PR also fixes live editor updates emitted by the Think agent:

• Generated file contents now appear when selected. File updates use the slash-stripped paths expected by the UI, such as src/App.tsx.
• Deleted files disappear immediately. Deletes emit a dedicated file_deleted WebSocket message instead of an empty file write.
• Commit and rollback updates refresh editor state. The editor file tree remains consistent with the restored workspace.

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