docs: clarify agent tool access and explore vs general distinction

[alanpog]

Summary

Fixes #9297

• Clarifies that primary agents don't automatically have access to all tools (Plan is restricted)
• Explains the key difference between General and Explore subagents: General has full tool access while Explore is read-only

Changes

1. Primary agents section: Replaced "can access all configured tools" with accurate statement about permissions
2. General section: Clarified it has full tool access (except todo) and can make file changes
3. Explore section: Added "read-only" and "Cannot modify files" to distinguish from General

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found