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

## 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 #169
- PR #345 (Cloud Run runtime + Tier-3 mount types)