Fix malformed workspace Git metadata handling

[HarperZ9]

This fixes malformed Git metadata handling in workspace scans on the current main implementation.

Before this change, an embedded NUL in a linked-worktree .git gitdir pointer, gitdir backpointer, or commondir value could raise during path resolution before the CLI emitted a workspace matrix. The CLI exited non-zero, but it printed a traceback instead of returning structured JSON.

The metadata path handling now rejects control characters before resolving paths and treats ValueError like other unreadable metadata failures. This preserves the existing linked-worktree authenticity checks on main, including gitdir backpointer validation and common worktrees parent validation. Malformed metadata is reported through coverage.diagnostics as UNVERIFIABLE, while healthy linked worktrees continue to scan normally.

Validation:

• python -m pytest -q tests/test_workspace.py
• python -m pytest -q
• CLI controls for malformed .git, gitdir backpointer, and commondir metadata: exit 1, JSON stdout, workspace_status: UNVERIFIABLE, empty stderr, and no raw control-character path fragments
• Healthy linked-worktree CLI control: exit 0, one MATCH repository
• git diff --cached --check -- src/public_surface_sweeper/workspace.py tests/test_workspace.py