fix: honor HTTP_PROXY / HTTPS_PROXY / NO_PROXY env vars (#35)

Node's built-in fetch (undici) ignores proxy environment variables, so
bl always connected directly and failed with ECONNRESET behind a VPN or
corporate proxy. Install an EnvHttpProxyAgent as the global dispatcher at
startup, but only when a proxy variable is actually set — behavior is
unchanged otherwise. Lowercase variables take precedence over uppercase
(curl convention) and NO_PROXY is honored.

Values are trimmed and passed explicitly to work around undici reading
env vars with ??, where an empty lowercase variable (https_proxy="")
masks a configured uppercase one. Invalid proxy URLs fail with a clear
usage error instead of a stack trace, and the ECONNRESET hint now
suggests exporting HTTPS_PROXY.

Tests are fully offline and need no credentials: unit tests cover env
parsing, and the e2e test runs a minimal probe (setupProxyFromEnv + a
bare fetch) against a .invalid host through a local CONNECT proxy to
verify traffic routes through the proxy, NO_PROXY is honored, no
dispatcher is installed when unset, and invalid values error clearly.

pnpm-lock.yaml

@@ -18,6 +18,9 @@ catalogs:
     chalk:
       specifier: ^5.6.2
       version: 5.6.2
+    undici:
+      specifier: ^8.4.1
+      version: 8.4.1
     vite-plus:
       specifier: latest
       version: 0.1.22
@@ -48,6 +51,9 @@ importers:
       chalk:
         specifier: 'catalog:'
         version: 5.6.2
+      undici:
+        specifier: 'catalog:'
+        version: 8.4.1
     devDependencies:
       '@clack/prompts':
         specifier: ^0.7.0
@@ -993,6 +999,10 @@ packages:
   undici-types@7.19.2:
     resolution: {integrity: sha512-qYVnV5OEm2AW8cJMCpdV20CDyaN3g0AjDlOGf1OW4iaDEx8MwdtChUp4zu4H0VP3nDRF/8RKWH+IPp9uW0YGZg==}
 
+  undici@8.4.1:
+    resolution: {integrity: sha512-RNHlB4fxZK0IrkhBsxhlbx7s8kFWwr7rzzOqj5nvZugw3ig3RsB7KW3zVlV0eu8POl+rx5d1hmL7rRg0z1owow==}
+    engines: {node: '>=22.19.0'}
+
   vite-plus@0.1.22:
     resolution: {integrity: sha512-fCCmEKjI+Hv74PdL/MKcrBkdYPHFNcqD5568KxwN0sa4SGxtcbs55i/577LxKs0w5zIjuLRZZ0zQPu9MO+9itg==}
     engines: {node: ^20.19.0 || >=22.12.0}
@@ -1750,6 +1760,8 @@ snapshots:
   undici-types@7.19.2:
     optional: true
 
+  undici@8.4.1: {}
+
   vite-plus@0.1.22(@types/node@24.12.2)(jiti@2.6.1)(typescript@6.0.3)(vite@8.0.10(@types/node@24.12.2)(jiti@2.6.1)(yaml@2.8.3))(yaml@2.8.3):
     dependencies:
       '@oxc-project/types': 0.129.0