#8156: Fix key formatting in memory configuration

fixing issue below ◇ Unknown config keys ─╮ │ │ │ - memory │ │ │ ├───────────────────────╯ <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adjusts the `FIELD_LABELS` map in `src/config/schema.ts` so the top-level `memory` config key is represented as a string literal (`"memory"`) rather than an unquoted identifier. This aligns the UI-hints key formatting with the dot-path strings used elsewhere (e.g. `"memory.backend"`), preventing `memory` from being treated as an “unknown config key” in schema/UI hint consumers. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is a single-key update in a static label map and is consistent with surrounding string-key patterns; there are no behavioral code paths affected beyond how a UI label is looked up. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->