feat: per-issue assignee adapter overrides (model, effort, workspace)

Add assigneeAdapterOverrides JSONB column to issues, allowing per-issue
model, thinking effort, and workspace overrides when assigning to agents.
Heartbeat service merges overrides into adapter config at runtime. New
Issue dialog exposes these options for Claude and Codex adapters.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/migrations/meta/_journal.json

@@ -148,6 +148,13 @@
       "when": 1772032176413,
       "tag": "0020_white_anita_blake",
       "breakpoints": true
+    },
+    {
+      "idx": 21,
+      "version": "7",
+      "when": 1772122471656,
+      "tag": "0021_chief_vindicator",
+      "breakpoints": true
     }
   ]
 }