WIP: UC Volume raw file sync persistence prototype

[nkarpov]

Summary

This PR captures the current raw-file UC Volume persistence experiment as a reference implementation.

Included

• terminal type persistence contract in type.json
• session env wiring for persistence config
• shared bootstrap restore/sync helpers using databricks fs
• claude/codex persistence config + launch integration
• app resource/env wiring for AGENT_STATE_VOLUME

Why WIP / not ready to merge

We are seeing practical concerns with this approach:

1. Startup latency is high

   • restore is synchronous and per-file
   • large trees (notably .claude/plugins) make CLI startup visibly slow

2. Operational complexity and correctness risk

   • many individual copy operations
   • harder to reason about consistency and failure behavior

3. Platform constraint remains

   • no native POSIX mount at /Volumes/... in app container
   • no usable FUSE path (/dev/fuse unavailable)
   • without a native mount, this approach may be fundamentally constrained

Current recommendation

Keep this PR as reference for discussion/iteration only.
Do not merge until we decide whether to:

• reduce scope and aggressively trim synced files, or
• switch to a different persistence strategy better aligned with runtime constraints.

[nkarpov]

Additional context from our investigation:

We also evaluated a snapshot-based strategy (single snapshot.tar.gz restore/sync) before switching to raw files.

Why we did not settle on snapshots:

1. Poor debuggability / observability

   • hard to inspect what changed between syncs
   • difficult to inspect specific agent artifacts directly in UC Volume

2. Coarse-grained updates

   • small edits require rewriting/reuploading the whole snapshot
   • less efficient for frequently-changing session state

3. Recovery and corruption blast radius

   • one damaged/missing snapshot affects all persisted state for that type
   • less graceful than file-level partial recovery

So while snapshots reduce operation count, they trade away inspectability and incremental behavior in ways that hurt this use case.