Make the GPT-6 executor low-only and lead with computer use

[Tryanks]

Follow-up to #364. Rewrites the bundled GPT-6 execution description and its surrounding guidance:

• Inherits the former Sol task list (scoped implementation, debugging with a reproduction, migrations, code review, data analysis, evidence gathering) and adds computer use as a headline strength, naming the observe/inspect/read and act/wait tools so UI-driving and eyes-on-screen verification are routed there first.
• Makes low effort mandatory for this profile: low beats the former Sol executor at xhigh on quality at a fraction of the token cost, so medium or above is never justified and only wastes money. The workflow prompt, DESIGN.md and both locale descriptions say the same thing.
• Migration: an untouched execution row still carrying the previous bundled GPT-6 text is refreshed to the new text; customized rows are kept. Covered by a literal-JSON test.

Checks: cargo fmt --all --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo test --workspace --locked pass locally on macOS.