Remove URL fetch capability from editor.js playground

[Copilot]

The editor playground could load arbitrary 3rd-party markdown files via fetch(), including via URL deep-links in the hash fragment. This is removed entirely — the editor now only accepts directly pasted/typed markdown.

Changes

• Removed fetch infrastructure: fetchContent(), toRawGitHubUrl(), contentCache, ALLOWED_FETCH_ORIGINS
• Removed URL-fetched samples: Dropped issue-triage, ci-doctor, contribution-check, daily-repo-status entries from SAMPLES (all were URL-only with no inline content)
• Simplified deep-linking: loadFromHash() now only resolves built-in sample keys; http(s):// hashes no longer trigger a fetch
• Simplified sample loading: loadSample() is now synchronous — no fetch path, no error handling for network failures
• Cleaned up stale guards: Removed the __url sentinel value from the sample selector change handler

[Copilot]

Pull request overview

Removes the editor playground’s ability to fetch and load markdown content from arbitrary URLs (including via hash deep-links), restricting sample loading to built-in, inline content only.

Changes:

• Removed GitHub/raw URL conversion, fetch logic, caching, and origin allowlist.
• Dropped URL-only samples and made loadSample() / loadFromHash() synchronous and sample-key-only.
• Simplified the sample selector change handler by removing the custom-URL sentinel path.

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[Copilot]

index.html still contains <option> entries for removed sample keys (e.g., issue-triage, ci-doctor, etc.). With the fetch path removed and SAMPLES now containing only hello-world, selecting those options calls loadSample(key) which immediately returns and leaves the UI in a confusing/broken state. Consider either (a) updating the sample selector options to match SAMPLES, or (b) adding a guard here that resets the selector to a valid key / shows an error when SAMPLES[key] is missing.