docs: clarify executor subagents inherit the parent model

[ifauzeee]

Fixes #14

The SKILL.md and README both claimed execute <plan> dispatches a cheaper executor subagent. In practice, neither Codex nor Claude Code spawns a cheaper model unless one is configured explicitly — subagents inherit the parent conversation's model.

Updated the claim to match actual behavior:

• README.md: "cheaper models do the execution" → executor subagent inherits your current model unless configured otherwise (tagline, diagram, usage list, how-to, closing-the-loop).
• skills/improve/SKILL.md: "Cheaper models do the execution" → subagent inherits the parent conversation's model; and the execute variant now says to configure a cheaper model explicitly when spawning if that's desired.

No behavior change — documentation only.