Add red-team security assessment across iOS, Android, and worker stack

[jamditis]

Motivation

• Capture a focused adversarial review of transport/auth, secret custody, and execution trust boundaries across Android, iOS (app + keyboard extension), and the worker backend.
• Provide prioritized, actionable remediation and post-fix adversarial test cases to guide secure hardening and CI checks.

Description

• Add docs/security/red-team-assessment-2026-02-21.md, a 196-line report with attacker models, seven findings (high-severity SSH trust and key-custody issues called out), remediation guidance, and suggested adversarial tests.
• Calls out platform-specific items: iOS host-key verification and Keychain-to-AppGroup key mirroring, Android ScpUploader API footgun, and worker fail-open approval / shell invocation risks.
• Includes a prioritized remediation plan and suggested regression/adversarial test cases to prevent regressions.

Testing

• This is a documentation-only change; no unit tests were modified or required.
• Verified the new file was staged and committed with git status --short which showed the new file, and inspected contents with nl -ba docs/security/red-team-assessment-2026-02-21.md | sed -n '1,260p'; both commands completed successfully.
• No automated unit/integration tests applicable for this docs-only PR.

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Codex Task