sandbox.additionalDirectories resolves absolute paths incorrectly (prepends ~/.claude/)

Bug Description

sandbox.additionalDirectories in ~/.claude/settings.json does not correctly resolve absolute paths. Absolute paths are prefixed with the ~/.claude/ directory, producing invalid paths.

Steps to Reproduce

1. Add to ~/.claude/settings.json:

"sandbox": {
    "additionalDirectories": ["/Users/username/.agent-memory"]
}

1. Start Claude Code
2. Run a Bash command that writes to /Users/username/.agent-memory/ (e.g., SQLite database write)
3. Command fails with "not writable" / sandbox permission error

Expected Behavior

The absolute path /Users/username/.agent-memory should be added to the sandbox write allowlist as-is.

Actual Behavior

The resolved sandbox config shows the path as /Users/username/.claude/Users/username/.agent-memory — the absolute path is joined relative to ~/.claude/ instead of being used directly. Relative paths (e.g., ../.agent-memory) also don't resolve correctly.

Environment

• macOS (Seatbelt sandbox)
• Claude Code (stable channel)
• additionalDirectories is not documented on the official sandbox docs page (https://code.claude.com/docs/en/sandboxing) or settings reference (https://code.claude.com/docs/en/settings), but it does have a partial effect (it appears in the resolved write allowlist, just with wrong path).

Workaround

Using "Allow unsandboxed fallback" mode — when the sandboxed write fails, Claude retries with dangerouslyDisableSandbox: true which goes through normal permissions.