chore: optimize AI agent context and implement CI cost controls (#5335)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

.gitattributes

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+# Mark only generated/derived XML as linguist-generated to reduce Copilot PR summary costs.
+# Hand-edited resources (layouts, string values) are intentionally excluded so they remain
+# visible in diffs and code review.
+**/composeResources/**/values-*/*.xml linguist-generated=true
+.skills/compose-ui/strings-index.txt linguist-generated=true
+
+# Ensure assets are treated as binary
+*.png binary
+*.jpg binary
+*.webp binary
+*.mp3 binary
+*.wav binary
+*.ogg binary