Stop shipping html-ppt README preview payloads

Constraint: README preview media and pre-rendered verification screenshots are nonessential for installed skills.
Rejected: WebP recompression | GIF conversion increased size and static compression leaves unnecessary package payload.
Confidence: high
Scope-risk: moderate
Directive: Use external source pruning fields instead of hand-editing generated plugin assets.
Tested: python scripts/validate_marketplace.py; validate_plugin.py plugins/codex/plugins/html-ppt; uv run python -m pytest backend/tests/test_skill_market_publish.py backend/tests/test_skill_market_scheduler.py -q; node web/tests/skills-market-ui.test.mjs; git diff --check
Not-tested: PowerShell execution because pwsh/powershell is not installed locally.

config/external-sources.json

@@ -289,6 +289,19 @@
         "README.md",
         "README.zh-CN.md",
         "LICENSE"
+      ],
+      "excludePaths": [
+        "docs/readme",
+        "scripts/verify-output"
+      ],
+      "readmeImageExcludes": [
+        "docs/readme"
+      ],
+      "readmeLineExcludes": [
+        "Every preview above",
+        "上图里的每一个预览",
+        "scripts/verify-output/",
+        "verify-output/"
       ]
     },
     {