docs(roadmap): add #368 — export always appends .txt; response.file reflects mangled path

[code-yeongyu]

Pinpoint #346

/export <path> always appends .txt regardless of requested extension.

Live on a1bfcd4:

/export /tmp/out.json  → creates /tmp/out.json.txt  (response: file: /tmp/out.json.txt)
/export /tmp/out.md    → creates /tmp/out.md.txt
/export /tmp/out.jsonl → creates /tmp/out.jsonl.txt

Pipeline reads response.file to open the exported file → gets wrong path when expecting .json. Makes machine-readable JSON export at a predictable path impossible without post-rename.

ROADMAP.md only