fix: apply agent-specific defaults to new sessions

[sudo-tee]

Resolve effective V2 model defaults and pass the selected agent, model, and variant into session creation. Preserve explicit creation options and keep V1 requests unchanged.

this should fix #532

[github-actions]

🏗️ Architecture check

Checks that require() calls between modules follow the layer rules in scripts/dependency-topology/topology.jsonc (entry → dispatch → services/UI → server/protocols; shared helpers usable everywhere).

Compared origin/main → HEAD.

	Before	After	Δ
Imports breaking layer rules	0	0	+0
Import loops	0	0	+0
Modules in largest loop	0	0	+0
Modules without a layer	0	0	+0

Imports between modules: +0 / -0

✅ No new problems

_{Run locally: make topology-check}

[stonemaster]

Thank you very much for this quick fix! I can confirm that it works for my setup ✔️