parser: add promptSource to LARGE_ROOT_FIELDS and remove the >32 KB head/tail workaround

[iamtoruk]

Follow-up to #1023 (partially addressed by #994, which added isSidechain to LARGE_ROOT_FIELDS).

parseLargeJsonl (src/parser.ts) still drops promptSource for lines > 32 KB, which is exactly where SDK-generated prompts live. #1021's recurring-context detector works around it by regex-scanning the raw line's head/tail (isMachineWrittenPrompt, src/optimize.ts). Proper fix: add promptSource to LARGE_ROOT_FIELDS (tiny scalar, add-only), then delete the head/tail workaround and rely on the parsed field. Add a > 32 KB fixture with promptSource: "sdk" to tests/parser-large-json-scanner.test.ts and assert the recurring-context detector still excludes it. Cache impact: none (promptSource is read at parse time; verify whether it needs to be persisted on CachedFile for the optimize path — if it does, that is add-only like isSidechain).