fix(forge): fail closed on aliases colliding on a routable host

Codex review (PR #520, P1): egress_forge_routes deduplicated referenced
forge accounts by hostname and silently dropped every account after the
first. Two aliases with different token_secret on the same host would let
all API calls to that host authenticate as whichever alias won the dedup —
acting as the wrong forge account and breaking the per-alias identity
guarantee.

Fail closed at composition (before the bottle is created): when two
referenced aliases resolve to the same host but disagree on
origin/auth/token_secret, resolve_forge_associations raises ManifestError.
Identical url/auth under two aliases still dedups to one route. The proxy
routes by host, so an ambiguous credential cannot be selected safely.

Regression tests cover both the conflicting-tokens (raise) and
identical-config (single route) cases; forge.py stays at 100% coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/egress/service.py

@@ -130,7 +130,12 @@ def egress_forge_routes(
     (`/api/v1`): the proxy injects the Gitea `token` scheme using the value of
     the host env var named by the account's `token_secret`, resolved at launch
     from the host environment — the token never enters the bottle. Aliases that
-    canonicalize to the same host share a single route (deduplicated)."""
+    canonicalize to the same host share a single route (deduplicated).
+
+    Composition (`resolve_forge_associations`) has already rejected referenced
+    aliases that share a host but disagree on origin/auth/token_secret, so this
+    host-dedup only ever collapses genuinely identical credentials — it never
+    silently discards a distinct one."""
     out: list[EgressRoute] = []
     seen_hosts: set[str] = set()
     for assoc in associations: