docs: clarify MCP gateway API key is leaked by design

[Copilot]

• Locate security architecture and MCP gateway documentation sections that describe gateway API key authentication
• Update security architecture docs to state that gateway API key inside agent container is not a strong security boundary and should be treated as leaked by design
• Update MCP gateway authentication reference with matching warning language for consistency
• Build docs with make build-docs and verify successful build
• Run make agent-finish before commit (fails on pre-existing unrelated testifylint issues in pkg/stats/spec_test.go and pkg/testutil/spec_test.go)
• Commit and push documentation updates
• Create/update PR description to close Safeoutputs write-sink reachable from bash in github-readonly profile — shared MCP bearer token bypasses declared read-only perm [Content truncated due to length] #26584

[pelikhan]

@lpcox

[Copilot]

Pull request overview

Updates documentation to clarify that the MCP Gateway API key available inside the agent container is not a strong security boundary and should be assumed compromised (“leaked by design”), aligning guidance with the security model discussed in #26584.

Changes:

• Add a warning to the MCP Gateway authentication reference noting the API key should not be treated as a secure lock against in-container code.
• Add a corresponding warning in the architecture/security overview describing the same limitation and recommended defense-in-depth controls.

Show a summary per file

File	Description
docs/src/content/docs/reference/mcp-gateway.md	Adds a warning in the authentication section clarifying the gateway API key is not a strong boundary once code runs in-container.
docs/src/content/docs/introduction/architecture.mdx	Adds an architecture warning reinforcing “leaked by design” framing and pointing readers to isolation/network policy/permission staging as primary controls.

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• Files reviewed: 2/2 changed files
• Comments generated: 0