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>

Services/DatabaseService.cs

@@ -197,6 +197,15 @@ public partial class DatabaseService
         TryAddColumn(conn, "sessions", "unity_mcp_token", "TEXT");
         TryAddColumn(conn, "messages", "tag", "TEXT");
 
+        // Migration: 1M-Suffix entfernen (Mai 2026). Seit März 2026 ist 1M-Context Default
+        // bei Opus 4.6/4.7/4.8 + Sonnet 4.6 – kein "[1m]"-Suffix mehr nötig.
+        try
+        {
+            conn.Execute("UPDATE agents SET model = REPLACE(model, '[1m]', '') WHERE model LIKE '%[1m]%'");
+            conn.Execute("UPDATE chains SET model = REPLACE(model, '[1m]', '') WHERE model LIKE '%[1m]%'");
+        }
+        catch { /* idempotent */ }
+
         conn.Execute(@"
             CREATE TABLE IF NOT EXISTS agent_skills (
                 id            INTEGER PRIMARY KEY AUTOINCREMENT,