docs(roadmap): add #335 — session list omits created_at_ms field

[code-yeongyu]

Pinpoint #335

Gap: /session list --output-format json session_details objects have no created_at_ms — only updated_at_ms.

Live on 0f7578c:

# session_details[0] keys:
["id", "lifecycle", "message_count", "path", "updated_at_ms"]
# created_at_ms: None ← absent

# id encodes creation time: session-1776891003038-0 → 1776891003038ms
# but this is undocumented and fragile

Without created_at_ms, callers cannot compute session age, distinguish fresh from stale sessions, or apply GC policies without parsing the ID string.

Fix: add created_at_ms to every session_details object; derive from session_meta event or ID timestamp; regression test.

ROADMAP.md only