This allows us to default to storing sandboxes locally rather than in ~/config/rwx, so that agents with strict permissions to only work in their current working directory can still access that file.

As an example, if you're using Codex and have this set in your config.toml, it cannot easily access the sandboxes file today:

sandbox_mode = "workspace-write"
approval_policy = "on-request"

This PR moves to always using a local .rwx/sandboxes/ directory, lazy-creating .rwx at the git root (or CWD) if one doesn't already exist.

Summary

• Sandbox storage always uses .rwx/sandboxes/sandboxes.json — global ~/.config/rwx/sandboxes.json support has been removed
• If no .rwx directory exists, one is created at the git repository root (or CWD if not in a git repo)
• The sandboxes/ directory is gitignored (via * pattern) and excluded from run uploads
• Locking simplified to a single local lock file
• Added GetTopLevel() to the git client for finding the repo root
• No changes to session key format or caller signatures in service_sandbox.go

Test plan

• Unit tests pass (go test ./internal/... ./cmd/...)
• Lint passes (golangci-lint run ./...)
• Manual: rwx sandbox start/exec/list/stop — verify sessions stored in .rwx/sandboxes/sandboxes.json
• Manual: Run from a repo without .rwx/ — verify .rwx is lazy-created at git root
• Manual: Verify .rwx/sandboxes/ is not uploaded with runs