Remove obsolete [1m] model variants

Since March 2026 the 1M-token context window is the default for Opus 4.6/4.7
and Sonnet 4.6 – no separate variant or beta header needed. The "[1m]" suffix
on model IDs is dead code now and would even produce HTTP 400 against the
current API.

Cleanup:
- Drop 3 [1m] options from the agent model dropdown; default selection moves
  from opus-4-7[1m] to opus-4-7.
- Drop 5 [1m] mappings from ResolveModelId and add a defensive Replace as a
  safety net for any agent record still holding an old value.
- One-shot startup migration on agents.model + chains.model strips the suffix
  so existing chains can be re-chatted without errors.
- ARCHITECTURE.md reflects the slimmed list and explains why 1M is now default.

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

wwwroot/index.html

@@ -201,12 +201,9 @@
         <select id="agentModel">
           <option value="sonnet-4-5">sonnet-4-5</option>
           <option value="sonnet-4-6">sonnet-4-6</option>
-          <option value="sonnet-4-6[1m]">sonnet-4-6[1m]</option>
           <option value="opus-4-5">opus-4-5</option>
           <option value="opus-4-6">opus-4-6</option>
-          <option value="opus-4-6[1m]">opus-4-6[1m]</option>
-          <option value="opus-4-7">opus-4-7</option>
-          <option value="opus-4-7[1m]" selected>opus-4-7[1m]</option>
+          <option value="opus-4-7" selected>opus-4-7</option>
           <option value="haiku-4-5">haiku-4-5</option>
           <optgroup label="OpenAI" id="openaiOptgroup">
             <option value="" disabled>Wird geladen...</option>