Hydrate a workspace

Agents read through the ODB; compilers need POSIX files. Hydration materializes the ws view — tip commit plus uncommitted WAL ops, matching what ws_read sees.

sturn ws hydrate agent-7 /tmp/ws-7            # whole view
sturn ws hydrate agent-7 /tmp/ws-7 --path src # subset

Or hydrate + run + clean in one step:

sturn exec --ws agent-7 -- cargo check        # tmpfs, gone after
sturn exec --ws agent-7 --keep -- cargo test  # keep the dir

• Target tmpfs (/dev/shm//tmp) — it's a projection, delete freely.
• Exit status propagates: sturn exec fails iff the command does.
• Writes inside the hydrated dir do not flow back — ops are the only write path. (Write-back hydration is a backlog item.)
• FUSE mounts for humans land last — git works meanwhile.