docs(prd-0017): nest auth.scheme + auth.token_ref under optional auth

Earlier draft had `auth_scheme: "none"` as the unauthenticated
signal — awkward sentinel. Nest the two credential-injection
fields under an optional `auth` key instead. Presence of the key
= authenticated; absence = unauthenticated. Empty `auth: {}` is
an error (omission is what means "no auth").

Touches: scope bullet, manifest example, mitmproxy addon
description's auth-handling step. Two trailing `auth_scheme:
"none"` references kept as historical context for what the new
shape replaces.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/prds/0017-egress-proxy-via-mitmproxy.md

@@ -21,7 +21,8 @@ are split between cred-proxy and pipelock:
    the cred-proxy URL.
 3. **Credential injection.** Continues cred-proxy's existing role:
    match by hostname (or hostname + path), strip inbound
-   Authorization, inject one based on `auth_scheme` + `token_ref`.
+   Authorization, inject one based on the route's optional `auth:
+   { scheme, token_ref }` block.
 
 Pipelock's role narrows to hostname allowlist + DLP body scanning
 on the egress-proxy → upstream leg. Pipelock no longer holds the
@@ -90,8 +91,10 @@ delivery.
   (was pipelock's CA).
 - Manifest rename: `bottle.cred_proxy.routes[]` →
   `bottle.egress_proxy.routes[]`. The route shape gains optional
-  `path_allowlist: [<prefix>, ...]` and supports `auth_scheme:
-  "none"`.
+  `path_allowlist: [<prefix>, ...]` and a nested optional `auth:
+  { scheme, token_ref }` block (presence/absence of `auth` is the
+  authenticated vs unauthenticated signal — replaces the old
+  `auth_scheme: "none"` pattern).
 - Agent's `HTTP_PROXY` / `HTTPS_PROXY` env vars repointed at the
   egress-proxy (was pipelock).
 - Pipelock retains its sidecar slot and its own DLP + hostname
@@ -151,30 +154,41 @@ default proxy path.
 ```yaml
 egress_proxy:
   routes:
-    # Authenticated route (today's cred-proxy shape, slightly
-    # renamed). path_allowlist optional.
+    # Authenticated route — `auth` block carries the injection
+    # config. path_allowlist optional.
     - host: "api.github.com"
-      auth_scheme: "Bearer"
-      token_ref: "GH_PAT"
+      auth:
+        scheme: "Bearer"
+        token_ref: "GH_PAT"
       path_allowlist:
         - "/repos/didericis/"
         - "/users/didericis"
-    # Unauthenticated path-filtered route.
+    # Unauthenticated path-filtered route — `auth` omitted
+    # entirely (presence/absence of the key is the auth signal).
     - host: "github.com"
-      auth_scheme: "none"
       path_allowlist:
         - "/didericis/"
-    # Bare-pass route: no auth injection, no path enforcement.
-    # Useful when you want a host to skip path filtering but
-    # still be DLP-scanned by pipelock.
+    # Bare-pass route: no auth, no path constraint. Useful when
+    # you want a host to skip path filtering but still be
+    # DLP-scanned by pipelock on the outbound leg.
     - host: "api.anthropic.com"
-      auth_scheme: "none"
-      # no path_allowlist → all paths pass
 ```
 
 Route matching is on `host` (was `path` prefix). The hostname
 gates whether a route applies; `path_allowlist` (if present)
-constrains the URL path under that host.
+constrains the URL path under that host. The optional `auth`
+block carries credential-injection config:
+
+- Omit `auth` → no Authorization header injected (replaces the
+  earlier draft's `auth_scheme: "none"`).
+- `auth.scheme` → one of `Bearer`, `token` (the values
+  cred-proxy supports today; sidesteps the gitea-token quirk).
+- `auth.token_ref` → host env var holding the secret. Same
+  semantics as cred-proxy's `TokenRef` field today.
+
+Validation: `auth` (if present) must contain both `scheme` and
+`token_ref`. An empty `auth: {}` is an error rather than a
+synonym for "no auth" — that's what omission is for.
 
 ### mitmproxy addon shape
 
@@ -187,9 +201,10 @@ The egress-proxy ships a small Python addon that:
   matches → forward unchanged (pipelock will hostname-gate it). If
   route matches and has `path_allowlist`, check `flow.request.path`
   against the prefix list; 403 with a clear reason if no match.
-- On approved requests: strip inbound Authorization, inject
-  `Authorization: <auth_scheme> <token-from-env>` if `auth_scheme
-  != "none"`.
+- On approved requests: strip inbound Authorization. If the route
+  carries an `auth` block, inject `Authorization: <auth.scheme>
+  <token-from-env-named-by-auth.token_ref>`. If the route omits
+  `auth`, leave Authorization unset.
 - SIGHUP / file-mtime watch on `routes.yaml` for hot-reload (same
   cadence as today's cred-proxy SIGHUP path).