CVE-2026-107716: banks may expose or overwrite any file via symlinks

### Summary
In `banks.registries.DirectoryPromptRegistry`, prompt file paths and the index file (`index.json`) do not refuse symbolic links. When a prompt directory contains or accepts untrusted files (e.g. unpacked archives, shared repositories, or multi-tenant folders), symbolic links pointing outside the registry root can be used to disclose arbitrary files via `_scan()` / `get()` or overwrite arbitrary files via `set()` / `_save()`.

### Details
Following PR #77, `DirectoryPromptRegistry` validates path resolution for prompt names. However:
1. `self._index_path` (`index.json`) is not checked for symbolic links. A symlink pointing to an external target (e.g. a configuration file) will be overwritten by `_save()` upon `reg.set()`, or read via `_load()`.
2. In `_scan()`, discovered `.jinja` files are opened and indexed without checking if `path.is_symlink()` or if the resolved path escapes the registry root. A symlink pointing to a sensitive file outside the root is read and indexed.
3. In `set()`, `prompt_file.write_text(...)` is called without checking if `prompt_file` is an existing symbolic link pointing outside the root.

### Impact
Arbitrary file disclosure (CWE-59 / CWE-200) and arbitrary file overwrite (CWE-59) in applications where prompt directories can be influenced by untrusted users or extracted from archives.

### Proof of Concept
```python
import os
from pathlib import Path
from banks.registries.directory import DirectoryPromptRegistry, DEFAULT_INDEX_NAME
from banks.prompt import Prompt

# Disclose external file via symlink in prompt directory
reg_dir = Path("/tmp/registry")
reg_dir.mkdir(exist_ok=True)
secret = Path("/tmp/secret.txt")
secret.write_text("SECRET_API_TOKEN")

os.symlink(secret, reg_dir / "leak.0.jinja")
reg = DirectoryPromptRegistry(reg_dir, force_reindex=True)
print("Disclosed content:", reg.get(name="leak", version="0").raw)

# Overwrite external file via symlink index
target = Path("/tmp/target.conf")
target.write_text("ORIGINAL")
(reg_dir / DEFAULT_INDEX_NAME).unlink(missing_ok=True)
os.symlink(target, reg_dir / DEFAULT_INDEX_NAME)
reg.set(prompt=Prompt("pwn", name="test", version="1"))
print("Target overwritten:", target.read_text())
```

### Remediation
1. In `_validate_index_path()`: verify `_index_path` is not a symlink and resolves within `_path`.
2. In `_scan()`: reject `path.is_symlink()` and check `path.resolve().is_relative_to(root)`.
3. In `set()`: reject existing symbolic links before writing.

A tested fix and regression tests have been prepared and pushed to:
https://github.com/jankesec/banks/tree/fix-directory-registry-symlinks-and-nesting