Rename SUBSD_* env vars to SUBS_* to match upstream refactor

The upstream SpacesOps app refactored its SUBSD_* environment variables to
SUBS_* (verified against server.js in spacesops/spacesops:v1.0.0). Update the
package to match so the configured values reach the app:

- main.ts: inject SUBS_URI_VALUE / SUBS_RPC_USER / SUBS_RPC_PASSWORD.
- configurePlatform action + storeJson: rename store keys subsd*→subs* and the
  field labels/descriptions (SUBS URL / SUBS RPC User / SUBS RPC Password).
- README, instructions, release notes: SUBSD→SUBS.

Note: the image's /app/setup-spacesops-env.sh still exports SUBSD_* defaults
(partial upstream refactor) — see report; vars the package does not set (e.g.
SUBS_RPC_URL) currently have no in-image default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

startos/fileModels/storeJson.ts

@@ -13,9 +13,9 @@ const shape = z.object({
   operatorRelay: z.string().nullable().catch(null),
   coingeckoApiKey: z.string().nullable().catch(null),
   coingeckoTokenCoins: z.string().nullable().catch(null),
-  subsdUrl: z.string().nullable().catch(null),
-  subsdUser: z.string().nullable().catch(null),
-  subsdPassword: z.string().nullable().catch(null),
+  subsUrl: z.string().nullable().catch(null),
+  subsUser: z.string().nullable().catch(null),
+  subsPassword: z.string().nullable().catch(null),
 })
 
 export const storeJson = FileHelper.json(