docs: add public feature flag config

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: gimenete

flags/feature-flags.json

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+{
+  "revision": 1,
+  "forceDisabledFlags": []
+}

flags/feature-flags.md

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+# Feature flag configuration
+
+[`feature-flags.json`](./feature-flags.json) is a global, disable-only list. `revision` must be a positive, monotonically increasing integer and increment with every payload change. `forceDisabledFlags` lists flag IDs to force off; an empty list restores normal resolution without enabling flags.