docs: correct security hardening facts, placeholders, and Docker mounts

[numnx]

Addresses a request to make security documentation precise and evidence-backed based on existing source code implementation.

• Substituted all instances of your_api_key_here with <YOUR_JULES_API_KEY> in docs-web/user/mcp-clients.md and .env.example.
• Fixed the typo --mcp-https-auth-token to --mcp-http-auth-token in docs-web/user/troubleshooting.md.
• Updated docs-web/architecture/security.md to specify X-Frame-Options: SAMEORIGIN and Permissions-Policy headers.
• Clarified Docker "Worker isolation" in docs-web/architecture/security.md and README.md to indicate specific explicit credential mounts instead of workspace mounts.
• Updated docs/operations/security-hardening.md to document the omission of framing and permission policies for preview-host responses.

Tests ran successfully across the backend, dashboard, and config tests without regressions.

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PR created automatically by Jules for task 4188206015953485370 started by @numnx

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