docs: remove collect from aggregation functions table

closes #659 (closed)

What does this MR do?

Removes collect from the aggregation function table in the design document and adds a note that it is planned but not yet implemented.

Why?

The design doc (docs/design-documents/querying/intermediary_llm_query_language.md) listed collect as a valid aggregation function, but attempting to use it returns:

aggregation[0] function "collect" is not supported

This caused contributor confusion : a user reading the design doc (the primary DSL reference) would naturally try collect and be surprised by the error.

docs/source/remote/queries/query-language.md already had the correct disclaimer. This MR syncs the design doc to match.

Changes

• Removed the collect row from the aggregation function table
• Added a note: collect (collect property values into an array) is planned but not yet implemented and will return an error if used

Verification

# Confirmed collect still errors (expected)
echo '{"query":{"query_type":"aggregation","nodes":[{"id":"v","entity":"Vulnerability","filters":{"state":"detected"}}],"aggregations":[{"function":"collect","target":"v","property":"severity","alias":"severities"}],"limit":1}}'   | glab orbit remote query --format raw -
# => ERROR: aggregation[0] function "collect" is not supported ✅

# Markdown linter
npx markdownlint-cli2 docs/design-documents/querying/intermediary_llm_query_language.md
# => Summary: 0 error(s) ✅