feat(memory): add owner-reviewed proposals

[gagan114662]

What changed

• Separates typed memory proposals from ordinary agent memory.
• Shows scope, evidence count, reason, and editable proposed content to the owner.
• Supports Approve, Reject, and Undo from the agent memory panel.
• Publishes approvals and review status as encrypted, versioned NIP-AE engrams.
• Limits writes to locally managed agents whose signing key is available.

Verification

• cargo fmt --all -- --check
• cargo check --manifest-path desktop/src-tauri/Cargo.toml -p buzz-desktop --lib
• pnpm --dir desktop exec tsc --noEmit
• Complete desktop Node test suite.
• Biome check on every touched frontend file.
• Desktop file-size, text-size, and pubkey checks.
• Vite production build.

Originating Buzz channel: 2db0f46d-71e4-46c2-9798-3b248dac5fff