docs: update native Windows hook guidance

[Lucas-FManager]

Fixes #2115

Summary

• Updated the Windows README section to document native Claude Code setup via rtk hook claude.
• Removed stale native-Windows CLAUDE.md fallback guidance from the supported-agents and troubleshooting docs.
• Added the current caveat that Claude Code PowerShell-tool mode is not rewritten yet and linked [FEATURE] Add Claude Code PowerShell hook compatibility #1319.

Root Cause

The docs still described the pre-v0.37.0 Windows fallback path even though the current hook initialization writes rtk hook claude for Claude Code on native Windows.

Validation

• git diff --check
• Targeted search over README.md and the two docs files for stale fallback wording and current rtk hook claude / PowerShell caveat wording.

Security

Documentation only. No hook code, shell execution behavior, settings writes, or command rewriting logic changed.

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