docs(roadmap): add #343 — session subcommand resume-safety inconsistently enforced

[code-yeongyu]

Pinpoint #340

/session fork hard-blocked at resume gate; /session switch+delete pass resume gate and reach argument parsing. Inconsistent allowlist.

Live on dc47482:

/session list   → success ✅
/session switch → {"error":"Usage: ..."} (resume gate passed, arg error)
/session delete → {"error":"Usage: ..."} (resume gate passed, arg error)  
/session fork   → {"error":"unsupported resumed slash command"} (hard blocked)

ROADMAP.md only