Track fd-anchored path hardening for docker workspace provider #114

Summary

The Docker workspace provider in src/coding_agent/environment/docker_workspace_provider.py currently uses Path.resolve(strict=False) containment checks in _validated_workspace_path(). This blocks straightforward path escapes but remains vulnerable to symlink-swap TOCTOU because later file operations reopen by path.

Why this matters

For stronger isolation, especially in compromised-container or untrusted-workload models, host-side file access should be descriptor-anchored instead of path-reopened.

Follow-up scope

• Replace path-based validation/access with fd-anchored traversal (openat/O_NOFOLLOW-style or equivalent)
• Apply consistently across read/write/replace/patch/glob/grep/cwd handling
• Revisit tests for symlink-swap race coverage

Context

Deferred from PR #113 to keep the Docker provider slice bounded to execution/timeout correctness.