Tasks: a default model per harness in `cli-config.yaml`

Each task names the requirement it satisfies and the testing-plan row that proves it.

Task list

[x] A1: vocabulary. modelchoice.default_model, the no-model-flag finding, schema property in both copies. Req: R1, R2.5 · Test: T1, T5 · Depends: —
[x] A2: dispatcher. _resolved_choice falls back to the harness's default. Req: R2, R5 · Test: T2 · Depends: A1
[x] A3: gate. Checklist tail and confirmation name the default. Req: R3 · Test: T3 · Depends: A1
[x] A4: probe. models check combinations include the default. Req: R4 · Test: T4 · Depends: A1
[x] A5: docs. Config reference, template, interactive-sessions.md, skill text. Req: NFR · Test: T8 · Depends: A2, A3
[x] A6: verify. T6–T8, evidence. Depends: A1–A5