fix: _validate_env_vars skips Anthropic-backend agents + missing sed escaping

- bin/disinto: Remove '[ -n "$base_url" ] || continue' guard that caused
  all Anthropic-backend agents to be silently skipped during validation.
  The base_url check is now scoped only to backend-credential selection.

- lib/hire-agent.sh: Add sed escaping for ANTHROPIC_BASE_URL value before
  sed substitution (same pattern as ANTHROPIC_API_KEY at line 256).

Fixes AI review BLOCKER and MINOR issues on PR #866.

bin/disinto

@@ -1789,7 +1789,6 @@ _validate_env_vars() {
     while IFS='|' read -r service_name forge_user base_url _api_key; do
       [ -n "$service_name" ] || continue
       [ -n "$forge_user" ] || continue
-      [ -n "$base_url" ] || continue
 
       # Derive variable names (user -> USER_UPPER)
       local user_upper
@@ -1809,7 +1808,7 @@ _validate_env_vars() {
         errors=$((errors + 1))
       fi
 
-      # Check backend URL or API key
+      # Check backend URL or API key (conditional based on base_url presence)
       if [ -n "$base_url" ]; then
         # Local model: needs ANTHROPIC_BASE_URL
         if [ -z "${env_vars[ANTHROPIC_BASE_URL]:-}" ]; then