Cloud Run runtime: support per-agent worktrees (SharingModeWorktreePerAgent)

[scion-gteam]

Summary

Support SharingModeWorktreePerAgent (per-agent git worktrees) in the Cloud Run runtime.

Context

PR #345 (epic #169, storage provisioning PR3) fixed the Cloud Run runtime to SharingModeSharedPlain. From CloudRunRuntime.provisionWorkspace:

Cloud Run uses a shared, plain workspace for now: the initial runtime scope provisions a single broker-side workspace per project. Per-agent worktrees (SharingModeWorktreePerAgent) are a follow-up once Cloud Run multi-agent lifecycle lands, so the mode is fixed rather than derived.

Work

• Once the Cloud Run multi-agent lifecycle exists (see the Admin API lifecycle issue), derive the sharing mode instead of hardcoding SharedPlain.
• Wire SharingModeWorktreePerAgent through the Cloud Run provisioning path so each agent gets its own worktree (the Tier-1 ProvisionShared already supports worktree creation under the advisory lock).
• Tests covering per-agent worktree provisioning on a Cloud Run-style backend.

References

• Epic fix(runtime/k8s): specify container on PodExec calls #169
• PR Storage provisioning PR3: Cloud Run runtime + Tier-3 mount types (issue 169) #345 (Cloud Run runtime + Tier-3 mount types)