refactor(vault): centralize _KEYS_FILE constant in vault/__init__.py

reader.py and writer.py each independently computed the same path via
pyra_home(). Single definition in __init__.py; both modules import it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pyra/vault/__init__.py

@@ -0,0 +1,3 @@
+from pyra.utils.paths import pyra_home
+
+_KEYS_FILE = pyra_home() / "vault" / "secrets" / "api_keys.json"

src/pyra/vault/reader.py

@@ -1,9 +1,7 @@
 import json
-from pathlib import Path
 
-from pyra.utils.paths import pyra_home, safe_chmod
-
-_KEYS_FILE = pyra_home() / "vault" / "secrets" / "api_keys.json"
+from pyra.vault import _KEYS_FILE
+from pyra.utils.paths import safe_chmod
 
 
 def get_key(provider_id: str) -> str | None:

src/pyra/vault/writer.py

@@ -1,9 +1,7 @@
 import json
-from pathlib import Path
 
-from pyra.utils.paths import ensure_dir, pyra_home, safe_chmod
-
-_KEYS_FILE = pyra_home() / "vault" / "secrets" / "api_keys.json"
+from pyra.vault import _KEYS_FILE
+from pyra.utils.paths import ensure_dir, safe_chmod
 
 
 def set_key(provider_id: str, api_key: str) -> None: