security: contain workspace path resolution

[harshitethic]

Problem

workspace_repo() joined a caller-controlled workspace_id directly under .workspaces and only checked whether the resulting path existed. A traversal such as ../outside could resolve outside the intended workspace root if a matching repo directory existed, expanding the filesystem/command-execution boundary used by /api/execute and /api/workspace.

Solution

• resolve the workspace root and candidate repo to canonical paths
• require the resolved repo to remain under the workspace root
• reject escaping paths with HTTP 400
• require the resolved target to be a directory
• add regression coverage for traversal rejection and normal in-root resolution

Validation

• python -m compileall app tests — passed
• python -m unittest discover -s tests -v — 25 tests passed
• git diff --check — passed
• reviewed the focused 2-file diff before push; no credentials or environment secrets introduced

Scope

This PR only hardens workspace path resolution and tests that boundary. It does not change command allowlisting, repository cloning behavior, or product features.

AI assistance disclosure: AI assistance was used to identify the path-containment issue, implement the focused fix, add regression tests, and inspect the resulting diff.