feat(secrets): encrypt egress tokens at rest with per-bottle ENV_VAR_SECRET

Implements the interim secret-provider design (PRD prd-new-secret-provider):
each agent receives a random ENV_VAR_SECRET injected into its container env
at launch. The host uses this key to encrypt each egress auth token value
(HMAC-SHA256 CTR mode, stdlib-only) and store it in a new
bottled_agent_secrets table (one row per env var, key column plaintext for
auditing). The key never touches the DB.

On infra container restart the in-memory token map is lost. launch_consolidated
now calls _reprovision_running_bottles after ensure_running: for each
registered bottle still alive on the gateway network it execs
`printenv ENV_VAR_SECRET` into the agent container and posts the result to the
new POST /bottles/<id>/reprovision_gateway control-plane endpoint, which
decrypts the stored rows and restores _tokens — no manual intervention needed.

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

bot_bottle/orchestrator/secret_store.py

@@ -0,0 +1,94 @@
+"""Symmetric encryption for per-bottle egress secrets (PRD prd-new-secret-provider).
+
+Each agent receives a random ENV_VAR_SECRET at startup — passed as an env var,
+never logged or persisted. The host uses this key to encrypt each egress auth
+token value before writing it to the bottled_agent_secrets table; the DB rows
+(ciphertext, plaintext env-var name) without the key are insufficient to
+recover the credentials.
+
+On orchestrator restart the in-memory token map is lost. The host-side
+reattachment path reads ENV_VAR_SECRET from the running agent container via
+``docker exec … printenv ENV_VAR_SECRET`` and posts it to
+``POST /bottles/<id>/reprovision_gateway``; the orchestrator decrypts the
+stored rows and re-populates ``_tokens``.
+
+Encryption scheme: HMAC-SHA256 used as a PRF in CTR mode (stdlib-only,
+no external deps). Each value is encrypted independently. The output blob is
+``nonce (16 bytes) || ciphertext`` encoded as URL-safe base64 (no padding).
+
+    keystream_block_i = HMAC-SHA256(key, nonce || i.to_bytes(4, "big"))
+    ciphertext_i      = plaintext_i XOR keystream_block_i[:len(plaintext_i)]
+"""
+
+from __future__ import annotations
+
+import base64
+import hashlib
+import hmac
+import secrets
+
+_KEY_BYTES = 32    # 256-bit key from ENV_VAR_SECRET
+_NONCE_BYTES = 16  # 128-bit random nonce per encrypt call
+_BLOCK = 32        # HMAC-SHA256 output width == one keystream block
+
+# Env-var name the agent container receives at startup.
+ENV_VAR_SECRET_NAME = "ENV_VAR_SECRET"
+
+
+def new_env_var_secret() -> str:
+    """Generate a fresh ENV_VAR_SECRET: 32 random bytes as URL-safe base64."""
+    return base64.urlsafe_b64encode(secrets.token_bytes(_KEY_BYTES)).rstrip(b"=").decode()
+
+
+def _b64dec(s: str) -> bytes:
+    return base64.urlsafe_b64decode(s + "=" * (-len(s) % 4))
+
+
+def _keystream(key: bytes, nonce: bytes, block_index: int) -> bytes:
+    return hmac.new(
+        key, nonce + block_index.to_bytes(4, "big"), hashlib.sha256
+    ).digest()
+
+
+def encrypt_value(secret_b64: str, plaintext: str) -> str:
+    """Encrypt a single string value with *secret_b64* (the ENV_VAR_SECRET).
+
+    Returns a URL-safe base64 blob ``nonce || ciphertext`` suitable for
+    the ``bottled_agent_secrets.value`` column."""
+    key = _b64dec(secret_b64)
+    pt = plaintext.encode()
+    nonce = secrets.token_bytes(_NONCE_BYTES)
+    ct = bytearray()
+    for i in range(0, len(pt), _BLOCK):
+        chunk = pt[i : i + _BLOCK]
+        ks = _keystream(key, nonce, i)[: len(chunk)]
+        ct.extend(p ^ k for p, k in zip(chunk, ks))
+    return base64.urlsafe_b64encode(nonce + bytes(ct)).rstrip(b"=").decode()
+
+
+def decrypt_value(secret_b64: str, blob_b64: str) -> str:
+    """Decrypt a blob produced by :func:`encrypt_value`.
+
+    Returns the original plaintext string. Raises ``ValueError`` for malformed
+    input or a key mismatch (wrong key produces garbage, not an error, unless
+    the plaintext is non-UTF-8 — treat all such failures as wrong key)."""
+    key = _b64dec(secret_b64)
+    try:
+        blob = _b64dec(blob_b64)
+    except Exception as exc:
+        raise ValueError(f"invalid ciphertext blob: {exc}") from exc
+    if len(blob) < _NONCE_BYTES:
+        raise ValueError("ciphertext blob too short")
+    nonce, ciphertext = blob[:_NONCE_BYTES], blob[_NONCE_BYTES:]
+    pt = bytearray()
+    for i in range(0, len(ciphertext), _BLOCK):
+        chunk = ciphertext[i : i + _BLOCK]
+        ks = _keystream(key, nonce, i)[: len(chunk)]
+        pt.extend(c ^ k for c, k in zip(chunk, ks))
+    try:
+        return bytes(pt).decode()
+    except UnicodeDecodeError as exc:
+        raise ValueError(f"decryption produced non-UTF-8 output (wrong key?): {exc}") from exc
+
+
+__all__ = ["ENV_VAR_SECRET_NAME", "new_env_var_secret", "encrypt_value", "decrypt_value"]